The problem is usually that the email address and/or registered name do not match exactly.
If you're still receiving the error, please do us a favor and login to your PayPal account separately, click on "My Profile" to check your registered name and email address as these must match exactly for PayPal to be able to connect. If you have a middle name or initial on your PayPal account, this must be entered on BrickLink.
If there is absolutely no discrepancy and you still cannot connect, please send us screenshots of your PayPal registered name and email address so that we may figure out what is causing the error.
Thank you so much and apologies again for the frustration this has caused.