How do I turn off Autocomplete in Outlook?

If you are using Outlook desktop client, follow these steps:

Select "Options" from the File menu.
Select the "Mail" tab, and then click the "Editor Options" button.
Click the "Advanced" tab, and then clear the check mark from "Show AutoComplete Suggestions."
Click Ok

For Outlook on the web, follow these steps:

At the top of the page, select Settings > View all Outlook settings > Mail > Compose and reply.
Under Text predictions, clear the “Suggest words or phrases as I type” check box.