Let's get this issue sorted out so you can create transactions and get back to business.
Being unable to create transactions due to unresponsive windows in QuickBooks Online can be caused by the stored data files on your browser. Web browsers gather cache files to speed up your browsing. However, too much can cause unusual behaviors like the one that you've experienced.
To fix this, I recommend accessing your account using a private window. It opens your account in a clean slate and loads fresh data.