2v60 Fixed Rate Shipping Method Error

Have downloaded and installed the fixed rate shipping method for 2v60
3 shipping methods defined (National, Across border and Collect from store)
Shipping methods made available in appropriate countries (ie Collect from store and National Delivery set for home country)
No discounts defined - on "Add to cart" the "Sorry we have a problem" page is displayed.

Error executing child request for handler 'System.Web.Mvc.HttpHandlerUtil+ServerExecuteHttpHandlerAsyncWrapper'.

System.Web.HttpException (0x80004005): Error executing child request for handler 'System.Web.Mvc.HttpHandlerUtil+ServerExecuteHttpHandlerAsyncWrapper'. ---> Nop.Core.NopException: Shipping rate computation method could not be loaded at Nop.Services.Orders.OrderTotalCalculationService.GetShoppingCartShippingTotal(IList`1 cart, Boolean includingTax, Decimal& taxRate, Discount& appliedDiscount) at Nop.Services.Orders.OrderTotalCalculationService.GetShoppingCartShippingTotal(IList`1 cart, Boolean includingTax, Decimal& taxRate) at Nop.Services.Orders.OrderTotalCalculationService.GetShoppingCartShippingTotal(IList`1 cart, Boolean includingTax) at Nop.Services.Orders.OrderTotalCalculationService.GetShoppingCartShippingTotal(IList`1 cart) at Nop.Web.Controllers.ShoppingCartController.OrderTotals(Boolean isEditable) at lambda_method(Closure , ControllerBase , Object[] ) at System.Web.Mvc.ActionMethodDispatcher.Execute(ControllerBase controller, Object[] parameters) at System.Web.Mvc.ReflectedActionDescriptor.Execute(ControllerContext controllerContext, IDictionary`2 parameters) at System.Web.Mvc.ControllerActionInvoker.InvokeActionMethod(ControllerContext controllerContext, ActionDescriptor actionDescriptor, IDictionary`2 parameters) at System.Web.Mvc.ControllerActionInvoker.<>c__DisplayClass15.<InvokeActionMethodWithFilters>b__12() at System.Web.Mvc.ControllerActionInvoker.InvokeActionMethodFilter(IActionFilter filter, ActionExecutingContext preContext, Func`1 continuation) at System.Web.Mvc.ControllerActionInvoker.<>c__DisplayClass15.<>c__DisplayClass17.<InvokeActionMethodWithFilters>b__14() at System.Web.Mvc.ControllerActionInvoker.InvokeActionMethodFilter(IActionFilter filter, ActionExecutingContext preContext, Func`1 continuation) at System.Web.Mvc.ControllerActionInvoker.<>c__DisplayClass15.<>c__DisplayClass17.<InvokeActionMethodWithFilters>b__14() at System.Web.Mvc.ControllerActionInvoker.InvokeActionMethodFilter(IActionFilter filter, ActionExecutingContext preContext, Func`1 continuation) at System.Web.Mvc.ControllerActionInvoker.<>c__DisplayClass15.<>c__DisplayClass17.<InvokeActionMethodWithFilters>b__14() at System.Web.Mvc.ControllerActionInvoker.InvokeActionMethodFilter(IActionFilter filter, ActionExecutingContext preContext, Func`1 continuation) at System.Web.Mvc.ControllerActionInvoker.<>c__DisplayClass15.<>c__DisplayClass17.<InvokeActionMethodWithFilters>b__14() at System.Web.Mvc.ControllerActionInvoker.InvokeActionMethodFilter(IActionFilter filter, ActionExecutingContext preContext, Func`1 continuation) at System.Web.Mvc.ControllerActionInvoker.<>c__DisplayClass15.<>c__DisplayClass17.<InvokeActionMethodWithFilters>b__14() at System.Web.Mvc.ControllerActionInvoker.InvokeActionMethodWithFilters(ControllerContext controllerContext, IList`1 filters, ActionDescriptor actionDescriptor, IDictionary`2 parameters) at System.Web.Mvc.ControllerActionInvoker.InvokeAction(ControllerContext controllerContext, String actionName) at System.Web.Mvc.Controller.ExecuteCore() at System.Web.Mvc.ControllerBase.Execute(RequestContext requestContext) at System.Web.Mvc.ControllerBase.System.Web.Mvc.IController.Execute(RequestContext requestContext) at System.Web.Mvc.MvcHandler.<>c__DisplayClass6.<>c__DisplayClassb.<BeginProcessRequest>b__5() at System.Web.Mvc.Async.AsyncResultWrapper.<>c__DisplayClass1.<MakeVoidDelegate>b__0() at System.Web.Mvc.Async.AsyncResultWrapper.<>c__DisplayClass8`1.<BeginSynchronous>b__7(IAsyncResult _) at System.Web.Mvc.Async.AsyncResultWrapper.WrappedAsyncResult`1.End() at System.Web.Mvc.MvcHandler.<>c__DisplayClasse.<EndProcessRequest>b__d() at System.Web.Mvc.SecurityUtil.<GetCallInAppTrustThunk>b__0(Action f) at System.Web.Mvc.SecurityUtil.ProcessInApplicationTrust(Action action) at System.Web.Mvc.MvcHandler.EndProcessRequest(IAsyncResult asyncResult) at System.Web.Mvc.MvcHandler.System.Web.IHttpAsyncHandler.EndProcessRequest(IAsyncResult result) at System.Web.Mvc.HttpHandlerUtil.ServerExecuteHttpHandlerAsyncWrapper.<>c__DisplayClassa.<EndProcessRequest>b__9() at System.Web.Mvc.HttpHandlerUtil.ServerExecuteHttpHandlerWrapper.<>c__DisplayClass4.<Wrap>b__3() at System.Web.Mvc.HttpHandlerUtil.ServerExecuteHttpHandlerWrapper.Wrap[TResult](Func`1 func) at System.Web.Mvc.HttpHandlerUtil.ServerExecuteHttpHandlerWrapper.Wrap(Action action) at System.Web.Mvc.HttpHandlerUtil.ServerExecuteHttpHandlerAsyncWrapper.EndProcessRequest(IAsyncResult result) at System.Web.HttpServerUtility.ExecuteInternal(IHttpHandler handler, TextWriter writer, Boolean preserveForm, Boolean setPreviousPage, VirtualPath path, VirtualPath filePath, String physPath, Exception error, String queryStringOverride) at System.Web.HttpServerUtility.ExecuteInternal(IHttpHandler handler, TextWriter writer, Boolean preserveForm, Boolean setPreviousPage, VirtualPath path, VirtualPath filePath, String physPath, Exception error, String queryStringOverride) at System.Web.HttpServerUtility.Execute(IHttpHandler handler, TextWriter writer, Boolean preserveForm, Boolean setPreviousPage) at System.Web.HttpServerUtility.Execute(IHttpHandler handler, TextWriter writer, Boolean preserveForm) at System.Web.HttpServerUtilityWrapper.Execute(IHttpHandler handler, TextWriter writer, Boolean preserveForm) at

Go to admin area > configuration > shipping >  shipping rate computation methods and ensure that you have al least one active method (click "edit" button and tick "is active")

BTW, where have you downloaded fixed rate shipping method for v60? It's already provided with nopCommerce (built-in)?

The "Shipping Rate Computation Methods" configuration page showed neither "Fixed Rate" or "By Weight" as active.
Attempts to change one to be active give the error "Error! The requested URL did not return JSON."
On further investigation the "Active" setting is successful despite the error - having now seen that only the Fixed Rate is set "Active" it appears to work OK and I can now add items to the cart!

The "Fixed Rate shipping" was not listed in the plug ins - searched around in desparation and noticed, on nopCommerce site, that the 2v5 was not compatible and only the multi option version was listed. I did a Google search and found a link to the nopCommerce site for the 2v60 version - that is the one I downloaded and installed (It says version 1v06 nopCommerce team). Can't remember link though.