Maya Good morning and welcome to the forum!
Unfortunately, there is little help I can provide: For security and privacy reasons all authentications are completely offloaded to Apple's system. There is literally not a single line in StitchBuddy's code prompting for credentials or having access to these - the app just requests macOS to perform an In-App purchase. So there is little to nothing for me to analyze…
I'd suggest to perform some basic steps like you did (including a reboot and double-checking your ID / password), and contact Apple's support if the problem remains… Sorry, I'd love to help in detail, but this process is beyond my control.