Same for me. iOS 15. BigSur 11.6. Xcode 13.0.

After restarting Xcode and iPhone multiple times and fighting against "Making the device ready for development", the issue disappeared. Now, I'm stuck on Profile trusting issue with iOS15. see https://stackoverflow.com/questions/69267550/ios-15-untrusted-developer-issue

Issue solved after disconnecting/reconnecting iPhone, waiting for Preparing both iPhone and Watch, and then enabling "Connect via network" on "Devices and Simulator / iPhone"