Translation:You should buy this shirt.
"You should probably buy this shirt" should be correct.
Not only did it mark my answer wrong, but it said the following answer was the correct one "You should TO buy this shirt"
No, “have to” describes the necessity objectively. 应该 is used for subjective wording.
Sometimes I really feel like I'm teaching remedial English phraseology to whoever is programming the answers for these lessons. And I'm not sure where they got their first lessons, sounds like out of a pretty bad textbook