Hello there @qnws1234,
Thanks for posting in the community!
That "authentication error" that you are seeing likely comes from a password in ShipStation needing to match up with what that corresponding system has on file. We see this with the connected Stamps.com account and see it resolved by navigating Settings>Shipping>Carriers and click the three dots next to add funds and choose update password.
Once that password is updated to match the Stamps.com account directly, you should see a return to normal service. Please feel free to reach out via Chat widget or email to our support team if this does not resolve the behavior you are seeing 🙂
I hope this helps!