I have two ideas that would fix the problem: do not work with the app project and Swift macro package at the same time. Only one of them can be opened. go to the issues navigator and find the issue that relates to 'Trust & Enable' the macro package. Is one of them working for you?

In my case I have got this error due to Aborting a request on Charles Proxy -> Breakpoints screen. It looks that Charles in such a case is returning null response

The code snippet in the question compiles in Swift 5.4.

It's due to the fact that the app template sets Target Deployment to iOS 14. To solve it, select the app target and select iOS15 in Deployment Info section.