Development NopCommerce version 4.2

I am creating backgound services that sould run in to timeline. i want to use IHostedService and Background Service but i am getting
an error
The type 'Task' is defined in an assembly that is not referenced .

Please help me anyone whith this.

Task are define in Nop.Services.Tasks
not to be confused with System.Threading.Tasks

Where is IHostedService I cannot find it in v4.2 ?

You will probably need to post some code to show us what you are trying to do