Hello,
The error message means that in your Shipstation user references does not work, now can you precise if you have this error in a erratic way, because if it's work and then suddenly don't work, there are something to investigate for sure.
But the first step is to check your Shipstation User references to eliminate the most obvious mistake.
And if the issue is deeper, can you active the Debug mode, and when you will have again this error message, provide your
log
file via our
Contact us
form (add an Url link to this subject in your message).
And try to provide any elements around your error that you can get in your merchant Shipstation account that can help us to progress on your subject.
Regards