While shipping with UPS you may receive this error:
HardError: The shippers shipper number cannot be used in the shipment.
If you search the internet for this:
"120121 the shippers shipper number cannot"
You'll see all the results point to the fact that the account is suspended for some reason. Possibly too many password attempts.
We contacted UPS support on behalf of a customer that was getting this error and the response was: "The error is usually caused when the account number is not active."
The only solution we have for this is to contact your UPS account rep and inquire as to the status of your account.