For what it's worth

I tried to save a Google Keep note with a customised repeat frequency - every 3 days. But the Save button on the reminder box was greyed out. Why? Was there a bug in the program?

No bug; it's very simple - Google Keep will not save a reminder frequency if the specified time (for the first reminder) is in the past. (Sensible, but it would be good to get a warning when you tried).