Follow this guide to add your UPS shipper number (account number) to your profile. This should solve the UPS error 120100: Missing or invalid shipper number error when using UPS shipping API or Postmen labels API.

Add UPS shipper number to

Log in to (Be sure to use the same User ID and Password that is in your Shipping API)
Select Account Summary from the My UPS tab rollover menu.
Verify your account is listed; if not, select the Add a UPS Account link
Follow the instructions to add the account
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