Spelling Errors are not underlined in Notes and Mail

Try checking in System Preferences >> Keyboard >> Text tab and Correct Spelling Automatically is enabled as per picture below. This is a System Wide setting including Apple Mail and Notes.

I think this might be caused Grammarly. Grammarly will disbale macos spellcheck as shown in this discussion thread. Turn spell check back on after Grammarly … - Apple Community. It wont revert back to original setting even after you uninstall it.

Type in terminal: defaults write -g NSAllowContinuousSpellChecking -bool true

This should fix the issue.

Im having the exact same issue. Ive checked my mail preferences (same as the OP) and Ive checked the system prefrences as you have suggested. I'm still showing no spelling correction suggestion (red line). However, when I right click the mispelled word it does show the correct spelling. This tells me that the system is recognizing the misspelings its not indicated with the red line. In fact, this appears to be the case regardless of the program im using. For instance, it did not recognize the mispelled word, mispellings but it did make a popup suggection to correct it.

Thanks!

Finally, it works Aprecited

Thanks for the hint!!!

Wonderful, thank you so much :)