You’re going to face increasing challenges with creating Mac App Store applications in Xojo, from both Apple and Xojo.
First
It appears that Apple have unofficially deprecated the Mac way of verifying receipts in a test environment. I have posted a solution in the following article, but I warn you now. It is API 1.0 only code and I’ve heard of several people failing to convert it to API 2.0.
https://ohanaware.com/blog/202151/Mac-App-Store-Receipt-Validation.html
Second
Apple now hide the built-in libraries and expect you to roll your own, the code above still uses existing Apple libraries, but in the future you’re going to have to look elsewhere.
Xojo actually include libCrypto within built Xojo apps, however they do not expose the functions needed for App Store receipt verification. There is a feature request for this, and I have lobbied for Xojo to implement this, but they felt we’d all like to re-write apps instead.
https://tracker.xojo.com/xojoinc/xojo/-/issues/64336
I wish you the best of luck, but in all honesty if your primary goal is to make Mac apps for the Mac App Store, I’d suggest you look into alternative Mac development tools for the future.