参考链接
'Use of Unresolved Identifier' in Swift
One possible issue is that your new class has a different Target or different Targets from the other one.
For example, it might have a testing target while the other one doesn't. For this specific case, you have to include all of your classes in the testing target or none of them.
https://learnappmaking.com/unresolved-identifier-understanding-xcode/