If using Google Chrome browser it is not ON by default
Got to settings (top right three dots)
Settings
scroll down and open Advanced Settings
Scroll down to find Languages, and see the Spell checker option...turn it on
There are quite a few of our terms I have to teach it like "venturi" and almost all Proper names
Until I told it how Ted Fancher and his Fancherized Twister are spelled the auto correct always made it Francher......grrrr