I got an error about Wordfence not being able to sign in as admin when trying to scan
We are aware of this problem with some sites and haven't tracked down the root cause yet. However, a workaround that works for many is to create another user with "admin" access. Make sure the username and password are extremely hard to guess, because no one will ever log in using this account. Then try to do a scan, and it should work.
If this helps and you want to help find the issue, please create a post at https://wordpress.org/support/plugin/wordfence, describe your setup and plugins, and include a link to this article.