.Net core web api -dotnet core 2.0

Try below coding

 

1. In Project settings > Player > Other settings, try to change the Api compatibility level to ".NET 4.x". That's what fixed it for me

2. Update Visual studio Version

this could be due to not all nuget packges are restored properly. i have created many web api using .net core which you can find many of my artciles.

https://www.c-sharpcorner.com/blogs/how-to-build-a-rest-apiwebservice-application-using-net-core-sqlback-end-and-htmlfront-end-with-java-script-to-consume-that-webservice

 

if this does not help, please upload the class where you are getting this error