I just upgraded to Scrivener 3 a few days ago, and autocomplete not working. I have a list of words entered, and have used this feature extensively in the previous version.
The preferences for Auto-Complete are in Preferences > Corrections. There are three options now under auto-completion: if you have ‘suggest completions as you type’ ticked, then the most likely problem is that it’s set to work only in script mode — make sure that’s unticked to work in the whole of Scrivener.
If it’s still not working, then let us know.
HTH