You shouldn't put the test project inside the main project, you should create a solution then link the main project "webapi" and the test project.

on your new folder create the test project using the commands it's more stable I'd say:

# dotnet new xunit -n "Webapi.Tests"

your main project should be in the same folder

WebapiProject
|__ Webapi
|__ Webapi.Tests

Note that you should create a solution and add them together into a single solution, also don't forget to add the main project reference inside the test project.