Mattermost Support channel (in mattermost you get direct interaction for your doubts, quick answers, anyways, answer is not guaranteed, sometimes people is busy)
TIP: Mattermost also have channels in Spanish, German and French, in case you feel intimidated chatting in english
General support forum (here you usually get answers, but is mandatory you prepare your question properly, please read Asking for Support page before)
TIP: There are also forums in other languages, like Spanish support forum (Foro en español), and German support forum (Deutsches Forum)