Did you go through the steps mentioned here? The Fakes framework and entire unit test subsystem was rewritten between 2012 and 2022. You need to make adjustments to your unit test projects (including upgrading them) as discussed in the docs before anything will work.
Once you've done that, and if it still doesn't work, then please provide us the details of your project including the target framework, SDK or legacy project, test framework and version being used, and screenshots and/or example code to replicate the issue.