autofill incorrect

I've already gone to Safari>Preferences>Autofill and deleted everything

What exactly does that mean?

That's not AutoFill; it's spell checking.

To remove a word you added to the spelling dictionary, type it, then right-click the highlighted word and select Unlearn Spelling from the contextual menu.

Try typing the word in a TextEdit window.

Do you use iCloud Drive, and if so, does the same thing happen on other devices linked to the same account?

It could just be because you've typed a certain word a lot of times, eventually your computer will think you want that word instead of something else.

Just in case there's a misunderstanding here, you can't right-click that little correction box in the screenshot. Turn off automatic correction, but leave spell checking enabled. Type the word that you want to remove from the dictionary, then do as I suggested earlier.

Triple-click the line below to select it:

~/Library/Spelling/LocalDictionary

Copy the selected text to the Clipboard by pressing the key combination command-C. From the Finder menu bar, select

Go ▹ Go to Folder...

and paste into the box that opens (command-V). You may not see what you pasted because a line break is included. Press return. A folder may open with an item selected. If it does, open the selected item in a text editor such as TextEdit.

open textedit - spell the word correctly - select the word - right click - "learn spelling" Done!

Worked for me with the exact same problem.

Try this if in trouble - it has worked for three of us.

https://discussions.apple.com/thread/6901735?start=15&tstart=0