I'll make sure that you can click the category box and pick the appropriate account quickly, @louise33. You're in the right place to solve this problem.
I'm pleased to provide troubleshooting steps to help resolve the problem you've encountered in QuickBooks Online (QBO). Thus, you may choose the account category by typing the name in the box.
It's not necessary to click the category dropdown arrow to select an account since you can type the name in the box and choose it from the list. See the sample screenshot below for your reference.
Since you're unable to do this in your account, I'd recommend logging in to your account through an incognito window. Encountering this kind of issue is typically the result of outdated or corrupted cache files in your web browser. By doing this, we can see if it's browser-related. You can use the keyboard shortcuts below:
Ctrl + Shift + N (Google Chrome)
Ctrl + Shift + P (Firefox)
Control + Option + P (Safari)
Ctrl + Shift + P (Microsoft Edge)
Once logged in, re-type the category name in the box and select it from the list. If it works, please return to your regular browser and press the CTRL+ Shift and Delete keys on their keyboard to clear the cache.
In addition, make sure that your internet connection is sturdy to prevent any delay in viewing.
Check out these articles below to know the different categories and how to handle these in QuickBooks: