Testing this, NUnit Test project doesn't let you switch between Framework and Core once you have created it. Perhaps because different PackageReferences are needed.
Safest fix is to create a new NUnit Test project. But this time, when it asks for target Framework, pick a .Net core one. Probably pick the highest number it offers (Core 3.1).
Then copy or move all your test source files into it.
Looking at a Framework Test Project vs a Core one, these are the lines that are different.
Framework:
Testing this, NUnit Test project doesn't let you switch between Framework and Core once you have created it. Perhaps because different PackageReferences are needed.
Safest fix is to create a new NUnit Test project. But this time, when it asks for target Framework, pick a .Net core one. Probably pick the highest number it offers (Core 3.1).
Then copy or move all your test source files into it.
Looking at a Framework Test Project vs a Core one, these are the lines that are different.
Framework:
Core: