I'm developing a live activity, and I had the same problem. I solved it by compiling the widget target before using it in the main target. You should try this in case the extension is unavailable in the main target. @hapiii @JeromeMF @philippnoah @turtlenerd

Another scenarie maybe be when the target membership of main target is unselected, for example: When i created the extension, by default the main target comes unselect, after turn selected works