We haven't received any reports from other users, like you, who are having issues when printing your receipts through your Merchant Services account.
Thus said, let's perform some browser troubleshooting steps so you can access your account without any errors. Let's start by logging into your account using a new private or incognito window of your browser. By doing so, it'll help us identify if the error message is related to your browser or not. To do so:
Google Chrome: Ctrl + Shift + N
Internet Explorer: Ctrl + Shift + P
Mozilla Firefox: Ctrl + Shift + P
Safari: Command + Option + P
Once you can access your account without any errors, you can clear your browser's cache so it can start from scratch. It'll also improve your browsing experience while working in QuickBooks.
However, if you still receive the error, I'd recommend contacting our Merchant Services Team. From there, someone will help you access your account successfully and create a case ID or ticket if needed to do so.