Delete any Program.cs
file that might have been automatically added to the project.
Remove:
<OutputType>Exe</OutputType>
Add:
<ItemGroup>
<PackageReference Include="Microsoft.NET.Test.Sdk" Version="16.9.4" />
<PackageReference Include="NUnit" Version="3.13.1" />
<PackageReference Include="NUnit3TestAdapter" Version="3.17.0" />
</ItemGroup>
Your final .csproj file should look something like:
<Project Sdk="Microsoft.NET.Sdk">
<PropertyGroup>
<TargetFramework>netcoreapp3.1</TargetFramework>
</PropertyGroup>
<ItemGroup>
<PackageReference Include="Microsoft.NET.Test.Sdk" Version="16.9.4" />
<PackageReference Include="NUnit" Version="3.13.1" />
<PackageReference Include="NUnit3TestAdapter" Version="3.17.0" />
</ItemGroup>
</Project>