I'm here to take care of your concern today, @vicky-hodge.
If you have the same login credentials for your company files in QuickBooks Online, all the regions/versions of QuickBooks will be displayed on the Please select the company you want to open window. All you need to do is choose a file you want to open from there. If not, then each of your companies has a different username and password.
On the other hand, if you're using QuickBooks Self-Employed, this message appears when you use US login credentials in the QBSE UK login (other regions) page. You can tap on the Continue button to see your US account.
Once you successfully log in to QuickBooks, you can skim through our help articles in case you need tips and related resources in the future.
If there's anything else you need help with, let me know by adding a comment. I'm always here for you. Have a good one!