Grammatical errors is what I use.
Kitty is right on her choice and recommendations
Both are correct. "Grammatical" is an adjective, but in English we--correctly--use nouns as adjectives too. "Grammar errors" is an example of that. "Grammar" is a noun used as as an adjective, just like "hair brush." "Hair" is a noun used as an adjective.
I don't get along very well with grammar cops, this isn't the place to go around correcting people. We all make mistakes regardless of education and intelligence, so lighten up and just have fun. Who cares if a word is misspelled or an answer is worded incorrectly? I don't come here for lessons in grammar or to take any exams :)
Grammatical errors is correct, because, if you wished to say it archaically, you would say "errors of grammar" or "errors in grammar". The first would require grammar to be put in the genitive case, which is Grammatical. The second, as there is no ablative case in English, is correct as is. "Grammar errors" has the word grammar in the nominative case, which, strictly speaking, should can not be used correctly.