Thanks for being a part of our community and for your post.
If you are seeing this error it is very possible that your connection and/or credentials with the selling channel have expired or been changed.
My recommendation would be to reconfigure the selling channel by following the directions listed in the help center article below.
If you still are seeing an error after you reconfigure will want to contact our support team as they might need to troubleshoot and potentially escalate this.
-Erin: Your Friendly ShipStation Community Manager