3609820 - Why does a supplier see "Your company or user account does not exist on [buyer's] website" after creating a new account or linking with an existing account?

Why do I get the message below when accessing the SAP Ariba Sourcing dashboard after creating a new account or linking it to an existing account?

Your company or user account does not exist on [buyer] site. [buyer] may have disabled the automatic synchronization of additional users from your organization, or there may have been an error when synchronizing your Ariba Commerce Cloud account with [buyer] site.

user registration, strategic sourcing, username length, access issue, event access, registration failure , KBA , BNS-ARI-SS , Strategic Sourcing , BNS-ARI-SS-EVT-LGN , Sourcing Event Registration/Login , BNS-ARI-GS-EVT-SRG , Guided Sourcing Event Supplier Registration , Problem