To work on an App UI, you must have a workspace; it's your ticket into the Designer. When you create a workspace, you must associate it with a Git repo—either one you create from scratch or, if you want to collaborate with someone who has already started configuring this particular App UI, a copy of the Git repo they've been working in.

In fact, if you know that someone else has already made changes to an App UI you're planning to configure, it's considered best practice to use the same Git repository in your workspace, rather than creating a new one.