Move project to older Xcode version
You already did what most people would suggest first, make sure the project format is set to a version that will open in an older version of Xcode. That is going to make finding a solution tougher so you have to provide more details for anyone to help you.
List the steps you took to create the project in Xcode 15, and be specific and detailed. What type of project did you create?
The problem was solved by removal of all build files in copied project, keeping sources only