This typically occurs when you have been registered by your U.S. payer under more than one email address, and one of those accounts already contains your SSN/ITIN.
If the account containing your SSN/ITIN is linked to a previous tax year, it may be locked or tied to existing tax documents. Removing the SSN/ITIN from that account in order to assign it to a newer account is generally not recommended. Any such change must be reviewed and approved by your U.S. payer after evaluating the affected accounts and tax years.
In most cases, the recommended solution is to merge the accounts.
Please contact your inviting institution or U.S. payer for assistance resolving this issue.