Email quickqrmaker@fastmail.com. That address reaches the person who runs the site, and it is the only way to get in touch. There is no contact form, no chat widget and no support queue.
A code that does not scan is the most useful thing you can report. Say what you put in, which options you chose, which format you downloaded and what happened when you scanned it. If you can, run the file through the tester first and include what it says. That is usually enough to reproduce the problem without any back and forth.
Corrections to the guides are equally welcome. If a page states a number or a claim you can show is wrong, send the page and the correction and it will be fixed or removed.
Questions about how your data is handled are answered by the privacy policy, but if something there is unclear or you want a fuller answer, ask.
Codes are generated in your browser and never sent anywhere, so there is no record of what you made. That means a code cannot be recovered, looked up, edited or deleted on your behalf, because there is nothing stored to act on. A static code also cannot be repointed after printing. If you need a code whose destination can change later, you need a dynamic code from a provider that runs redirects, which this site does not do.
This is a small site run by one person, not a company with a support desk. Mail is read and answered, but not on a schedule, and not instantly.