That is not an error; that is a message from the debugger telling you that you cannot inspect a particular variable unless you allow all threads to run.
There will be a small icon that looks like twisted threads next to the message or the variable. Click that, and Visual Studio will evaluate the variable's value.
There's an explanation of this behaviour in an archived MS blog from 2012:
Why do we get “The function evaluation requires all threads to run” | Microsoft Learn[
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