Hey there, @youngsquarries-h. Let me share some troubleshooting steps to isolate the issue. I'm here to ensure you can apply a GST to your transactions.
Unresponsive or unclickable buttons or fields are usually a result of too much accumulated historical data causing this unusual behavior. Whenever you visit a website, it saves some information from the web page in its cache and cookies. These files allow the websites you visit most often to load faster. Not to worry, clearing them will do the trick.
First, let's try accessing your account through an incognito window to rule out the possibility of a webpage issue. You can refer to these shortcut keys to open an incognito window in all supported browsers:
- Chrome: Ctrl+Shift+N
- Firefox: Ctrl+Shift+P
- Safari: Command+Shift+N
If it allows you to select a GST (as shown in the screenshot below), switch back to your regular browser. Then, clear its cache. This way, you can access QuickBooks with a clean slate. You can also use a different supported one.

Additionally, if you need to move a transaction to a different account, undo or unmatch it. Then, categorise it again. Make sure to do all of your changes before your monthly reconciliations.
If you need further assistance with your banking transactions, you can post them here. I'd be happy to answer. Have a good one!