The remember me checkbox is no longer able to be checked on Safari. This happened quite recently, so I think it began with Safari 12.
I have tried turning off both the “Block all cookies” privacy setting and “Prevent cross-site tracking” settings in Safari, no no avail. Duo will complain that “you need to enable cookie to use remember me”
The issue is specific to Safari. Remember Me still work on Firefox and Chrome on my Mac.
Safari users are reporting this issue with other sites too, not just Duo. It is possible that the issue is related to the version of the Mac OS or iOS, not the version of Safari, as someone on this thread suggests https://discussions.apple.com/thread/250603517.