Mutteivat is incorrect. Mutteivät is correct (mutta eivät)
You can say mutteivat! Both are correct, mutteivät is probably just more common
Gosh, I'm learning something new about my native language while checking out this course...
I'd never use mutteivat myself, but it looks to be only somewhat less common than mutteivät (according to how many times Google finds each of them).
Mutteivat is wrong
Check out the link Anna posted above.