You can see here:
Lines 123 to 175 have numerous checks in the code for:
- Line 123 = Address not provided at all
- Line 135 = Address is too short (<= 2 lines), it needs to be formatted as multiple lines rather than comma seperated.
- Line 154 = Postcode format is invalid, you can check the postcode checker here: https://regexr.com/4on0o
This was written 2 years ago so the postcode format may have changed, part of the issue is that the specific error is not shown in the web interface, but only in an error log, this means people get an invalid address error, rather than one that says what is wrong with the address, i.e. which check failed.
I can fix this quite easily if the directors wanted. It seems to me like it has been 2 years since this system was updated and work is needed to address common issues.