Translation:Usually I visit my grandparents on Sundays.
"Usually ... on Sunday" has the same meaning as "on Sundays." Both imply habit, and the former is more common in English, I would believe.
I agree that on Sunday should be accepted. Another instance of Duo inconsistency as “on Sunday” is often listed as correct as a translation for on Sunday
If it doesn't accept I usually, then you should report it. Just saying. Even though I I'm doing this for another person who might come on here.