Remote access to IMac

I am trying to get access to my Dad's IMac and followed all of the steps in the post about doing this for Catalina https://community.teamviewer.com/t5/Knowledge-Base/How-to-control-a-Mac-running-macOS-10-14-or-higher/ta-p/44699 . However, when I try to remote in I am told I cannot connect. I think there is a window popping up on the IMac asking him to allow access. I am trying to avoid that happening so I can remote in because he cannot see well and does not have the coordination to click on the pop up window. Aside from the settings in the post, is there something else that has to be done at his end? Is there some sort of setting in the Teamviewer app? Does he have to have a separate TeamViewer account? Please help