May I know if you've encountered any error messages when attempting to print checks? I'd appreciate it if you can provide more info about this so I can give you an accurate resolution to resolve the issue.
In the meantime, let's perform some browser troubleshooting steps so you can be able to print checks in QuickBooks Online (QBO). To start, access your QBO account using a private browser. Private mode prevents storing cache and cookies that might cause problems when running Intuit sites.
Here are the shortcut keys you can use:
Google Chrome: click Ctrl + Shift + N
Mozilla Firefox and Microsoft Edge: press Ctrl + Shift + P
Safari: hit Command + Shift + N
If you don't experience the same thing, go back to your regular browser and clear the cache. This will remove the stored cache or cookies to refresh your browser's background process. If you still can't print checks, use another supported browser. It could be that the one you're currently using has a temporary issue with QuickBooks.