Built-in spell checker is unavailable in QuickBooks Online, Fsiw1919. We'll have to check the language preference of your browser. I'll guide you with the steps.
For Google Chrome:
For Internet Explorer:
Once done, log out from QuickBooks Online, then log back in. This will help the system to load the settings you've set up.
This is a good feature, though! I'll surely share this to our management team. They'll submit this to our software developers for review.
You can keep in touch if you need anything else.