Andrea The "StitchBuddy Creator" In-App purchase is inactive as it requires the Pro product, please refer to this FAQ article for more details about purchase options.
Regarding the issue you mentioned: On macOS In-App purchases cannot be performed in the App Store, but only from the app itself. Please try to reboot your Mac and double-check your account hasn't any pending agreements etc. to be approved. If the problem still exists I can provide some detailed instructions to investigate further, including some logged information. Maybe you'd like to contact me by email if required?
Edit: I propose to start new discussions for specific problems, making it easier to locate relevant information, e.g. for others facing the same situations…