If I do not do well in the quizzes does that mean I don't know the language?
I took the quizzes and did not do well and does that mean I did not know the lanugage I spend months studying.
If you mean the plus quizzes, don't worry, they test you out of all the things taught in the course and from what I've seen on the forums, they sometimes give you low ratings (2.5 out of 4 (or 5?)) even if you get everything right.
If you mean the placement test, testing out of singles skills or the lessons in general, it's possible you don't know the vocabulary duolingo teaches like Ccf-Uk_2018 said or that you are simply using expressions and grammatical forms that haven't been entered as correct alternatives by the course contributors. It happens even in well-developed courses, and Scottish Gaelic is only in beta, so it's bound to happen more often.