Fixed by following the steps provided by a helpful woman on the phone helpdesk.
The source of the problem:
In 2014 (over four years ago), I sent a test payment of $60 to the unvalidated, secondary email address, but never accepted it. The helpdesk could not delete the old request, and neither could I, because it was over a year ago.
The solution:
1) Delete the unconfirmed email address from your master PayPal account.
(The unconfirmed email address is a secondary email address, not the primary.)
2) Create a TOTALLY NEW PayPal account using the unconfirmed email address
3) Confirm that email address, which is now associated with the new account.
4) Close the new account. At that point, PayPal will cancel any pending transactions on that email address (originally, the unconfirmed one).
5) Go back to the first account, add the email address to it as a secondary email address
6) Confirm that secondary email address
