Open Keychain Access located in /Applications/Utilities
Select Passwords on the left.
Type the name of the website in the search field top right corner of the window.
Then right or control cliick that keychain then click Delete.
Now back to Safari > Prerferences > Autofill
Make sure: User Names and Passwords is selected.
And again, Safari > Preferences > Privacy
Click: Remove All Website Data (cookies) < won't hurt to do this one more time
Quit and relaunch Safari.
Sign in to the site. Click Yes when prompted to save the login data to a new keychain.
Keep in mind, only passwords are stored, not user names.
I deleted the password in the keychain, autofil is selected in Safari Privacy, Preferences. I could not find a cookie for this site. I closed the web psge, quit Safari, relaunched whet to the web site and the old/wrong user name still appeared. I did not remove all cookies. Does Keychain control User Ids and Passwords?