To give you the right amount of information, may I know what specific error are you having when trying to print in QuickBooks Online (QBO)? What specific transaction or report are you printing?
For now, we can try printing your transactions or reports in a private window. Sometimes, the regular browser tends to hold onto information, and over time it could cause problems while working with QuickBooks.
To save you time, use either of these keyboard shortcuts:
Mozilla Firefox: press Ctrl + Shift + P
Internet Explorer: press Ctrl + Shift + P
Safari: press Command + Shift +N
If you're able to print fine in a private window, go back to your original browser and clear its cache to start fresh. Also, using other supported browsers can help us narrow the cause of this behavior.
If you're getting the same problem, I recommend checking these articles to resolve printing issues in QBO: