C# Registering Delegate in Interface throw Target Framework Error

I am working on Azure Function FunctionA that is TargetFramework netcoreapp3.1. This FunctionA calls another project for services whose TargetFramework is netstandard2.0. In these service I have interface where I am trying to register Delegates but unable to do so and getting version error; reference below;

public interface ICustomerServices
{
    delegate Task<IEnumerable<string>> AppCustomerListHandler();
    Task<string> GetMyName();
}

Can I solve this problem without upgrading any framework

>Solution :

A delegate just describes the "shape" that some methods will conform to. It doesn’t, in itself, describe an actual member of any enclosing type. If you want to consume a delegate within an interface, that’s not a problem, e.g.:

public delegate Task<IEnumerable<string>> AppCustomerListHandler();
public interface ICustomerServices
{
    event AppCustomerListHandler AppCustomerListHandler;
    Task<string> GetMyName();
    void DoSomething(AppCustomerListHandler appCustomerListHandler);
}

Is just fine.