"These are not my friends."
Translation:Éstos no son mis amigos.
They shouldn't have. The accent mark used to be used to denote the difference between demonstrative adjective, "Estos amigos - These friends", and demonstrative pronoun, "Éstos son amigos - These are friends."
As Prof. Antonio says, nowadays the accent isn't needed unless there is a need for clarity. In this DL sentence "Éstos" is clearly a pronoun, so the accent is redundant. While not an error as such, DL should probably change it to "Estos", and they definitely shouldn't warn about a missing accent mark.