Manage Nuget Packages for Nop.Admin and dowgrade System.Web.Mvc to Version=5.2.2 and Clean, Build Solution.  After that, you will get bunch of error messages.

Manage Nuget Packages for Nop.Admin and upgrade System.Web.Mvc to Version=5.2.3 back. Clean, Build Solution.  After that, it will be compiled without error. Also, intellisense is working, again.

First I try to upgrade System.Web.Mvc to Version=5.2.4 and downgrade to Version=5.2.3, no result. But if you downgrade and upgrade it, voila.