When adding a split deposit eg $50 to sales and $5 for rebate, the GST code is not being selected and I can't enter it from the drop down list or by tabbing as I used to.

Hi RT-Books,

Thanks for posting in the Community! 

QuickBooks Online uses browser's cache and cookies to run faster. To isolate on what might be causing this issue, let's do some troubleshooting on your browser. Let's access your QuickBooks Online account using private browser to see if we get the same result.

A private browser does not store any activities, which is why it's a good idea to use it for any unexpected behavior in the program.

Here are keyboard shortcuts you can use depending on the browser you're using:

Google Chrome: press Ctrl + Shift + N
Mozilla Firefox: press Ctrl + Shift + P
Internet Explorer: press Ctrl + Shift + P
Safari: press Command + Shift + N

If you're able to assign a GST code, kindly go back to your default browser and clear your browser’s cache. It is a good idea to clear cache, to remove the temporary internet files and to help the application function properly. Please consider using different browser too.

For your reference, you can also check on this article: Categorise and match online bank transactions in QuickBooks Online. 

Feel free to drop a comment below if you have any other questions. Have a great day!