Sorry to hear about the trouble. Could you please confirm the version of Adobe XD and the Operating System you are using and also is this happening on a specific file or multiple files? Will it be possible for you to share a screenshot of the Property inspector when you try enabling the co-editing feature?
Thanks for sharing the details, Ren. Could you please check the resolution of the Window, it looks like you're at a very small screen resolution, likely lower than our minimum screen recommendation. Please change your zoom level on Windows to a smaller percentage - the enable coediting button should be at the bottom of the screen you're showing. It only shows once, so once you click on it for any document, it should be higher up on the screen for you to select on your original zoom settings.