Hello James-S
This error message simply states that the day on which you are trying to enter attendance is not a day where the course period (see Course Period days configuration) is taking place / or a school day at all (see green Calendar days and Marking Period dates).
Have you taken the Quick Setup Guide yet?
If the problem persists, please post screenshots of your problem here (Course Period configuration, Take Attendance screen, current Marking Period screen, and Calendar screen).