"Error Verifying Serial Number" message when enrolling a generator?

Issue:

When attempting to enroll the generator via Fleet or under the end-users account, you receive an error of "Error verifying serial number" or "invalid serial"

Example is from Fleet:

Possible Causes: 
1. There is an internal note in our registration system involving that serial number

2.  Potentially suspended warranty issues

3. The registration system is experiencing issues (where the serial numbers are pulled from)

Fix:

Please contact warranty or customer support for assistance in resolving issues