Keychain

See the articles Autofill your user name and password in Safari on Mac - Apple Support

Change Passwords preferences in Safari on Mac - Apple Support

If AutoFill doesn’t work in Safari on Mac - Apple Support

If AutoFill still doesn’t work, the website may not allow it.

To turn AutoFill off, choose Safari > Preferences, click AutoFill, then deselect the information you want to enter manually.

Note : you don't need to delete all passwords from keychain access , it's very risky the Mac keychain will get corrupt .

The less invasive way would be to clear the cookies for that specific website on your Mac.

1. In the Safari menu, click Preferences
2. Click Privacy
3. Click Manage Website Data
4. In the Search field, type in the name of the website that gives you troubles
5. The search result may return multiple hits. If they all look form the same base website, click Remove All. Otherwise, select the one website and click Remove