Matres non sellae sunt?
It should be correct. Try to report it the next time.
My children think their mother's lap is the safest seat possible.
I used just to check it out, Matres sunt non sellae. The system rejected it. The system wants to negate the copula rather than the nouns, which is objectionable.
Isn't chairs accusative here? Shouldn't it be sellas the plural accusative? Thanks.
No. "To be/esse" is a stative verb (verb of state) and not an active verb (verb of action). There is no action to be received. There is no object, direct or otherwise. "Sellae" is the plural nominative as it should be as the subject complement.