Cocoa finds Syaro handing out fliers for her part-time job. Syaro says she cannot return until she finishes handing out all the fliers.
全部 配り 終えるまで 帰れないわ」
A verb followed by まで expresses the condition of “until the action occurs”. The verb preceding まで is in its plain/dictionary form.
If the sentence is negative, the verb before まで must be completed before the following verb may occur. If the sentence is not negative, the verb before まで was completed before the following verb occurred.