Invalid Answer - You must complete the question before submitting (Pick Many)

Had the same issue - make sure when you select the correct answer in form view, you select the object/shape the user will click/select rather than the text of the right answer.

I had a button set for the user to click on to select the correct answer, however when i selected the correct answer in the form view i chose the text of the answer rather than the actual object/shape - which kept giving me the error message - Invalid Answer - You must complete the question before submitting.