USPS website is extremely particular on how the address is entered in the form. If there's an error on the website stating:
your US Global Mail address is invalid,
your US Global Mail address doesn't exist, or
you need to confirm that the establishment has been there for more than six months
Here's what do to:
When adding the first line of your address, please ensure there are no special characters in the form: no # signs, periods, or commas. The only accepted format is below: Street Address PMB NNNN
for example: 1321 Upland Drive PMB <your box number>
To find to your box number : log in to your account and find it in the left bottom corner of the screen.
Once your change of address is completed, do keep the confirmation code issued upon completion of your request somewhere safe. It will be needed if you ever want to view, edit or cancel this change.