For your first line of troubleshooting, you should consult the FAQs. However, if they have yielded nothing, try the general steps below before submitting a bug report. Due to the large scope of errors needed to be dealt with, these steps will be vague and widely applicable, and aim to achieve an optimal environment for GUI Pie to run in, rather than trying to address any preconceived problems.
Attempt to reinstall GUI Pie or get the latest update.
See if running GUI Pie as an administrator solves any issues.
Run GUI Pie from a different location. Try running it in a standard directory - not a cloud location or a shared drive.
Try recreating your error with a new save file. Create a blank file and see if you can force the same error using that file. If not, then the issue is due to your save file. It may be corrupted or faulty. If so, please send a bug report with a copy of this save data file through the user testing page, with all sensitive data omitted, and recreate your screen in a new file.
If possible, see if the error exists on a different machine, or is localised to your machine.
Ensure you are running Python 3.
Ensure you haven't changed the name of your Python file. Its name should match the name of your GUI object inside that file. If it is different, change it so that both are the same.
Update Python.
Ensure that the syntax of your script is correct, and you have successfully imported your GUI file, and that it is being run on the final line of the main file.
If your file is a group, ensure that you have shown at least one page.
Refer to the troubleshooting present in the Python documentation here.