I’m trying to turn off the word suggestions that appear below the line as I’m typing, because they distract me as I’m writing. I’ve tried Preferences > Corrections > Uncheck “Suggest completions as you type”, but that doesn’t seem to do anything. I’ve unchecked everything below “Suggest completions as you type”, and then unchecked “Suggest completions” too, and then rebooted Scrivener, and that didn’t help either. I looked in the Project Settings area and the Auto-Complete list is completely blank. Any ideas how I can disable this feature? Thank you!
System settings…?
Turn off Edit > Spelling and Grammar > Check Spelling While Typing.
Thank you! That seems to have worked. It stinks that I guess Scrivener won’t autocorrect spelling errors now, nor notice them, but it’s good that the annoying word suggestion popups are gone. What a choice, there. No popup words = no spell check! 
Thank you so much for letting me know how to fix those popup word suggestions. I appreciate the help!
Your Auto-Completion preferences aren’t acting normally. You should be able to use Check Spelling without seeing the suggestions. I’d try resetting Scrivener preferences as described in this L&L Knowledge Base article:
scrivener.tenderapp.com/help/kb … references
As the article notes, make sure at the start to save your current preferences through the Manage dropdown (and backup, of course).
We’re not talking about auto completion here. We’re talking about macOS’s ordinary spell check while typing, which you can see in Textedit, et al. With respect, @starsky confused the little Mac pop-up (which has an “x”-shaped close button at the left and only ever provides one option) with Scrivener 's auto completion (which, unless you’re doing a screenplay, is off by default and requires the user to provide a list of items to be completed.)
There are two types of prompt that can come up while you type. Scrivener’s is referred to as “completion”, which is tied into the Project Settings pane, under Auto-Completion. By default these will never appear unless you specifically call upon them, with the Edit ▸ Completions ▸ Complete menu command, or if you use Scriptwriting mode. The appearance of this function is like a menu that pops down from the word, in vertical format, where you can arrow between options and press Return to select one.
The other type of prompt comes from macOS, and that looks more like an iOS type thing, with a little blue bubble that appears and generally only provides one option to choose from. That I believe can be disabled system-wide in the System Preferences: Keyboard: Text pane, with Correct spelling automatically.
I managed to get confused by the original post. I should’ve asked. If it is just a matter of check spelling, Starsky and any onlookers, ignore my earlier post about resetting preferences.
I’m sorry for any confusion I might have introduced.
No problem, scshrugged! My response came across as a bit curmudgeonly, for which I apologize.