Change devices on backup plan is greyed out

Sorry I can’t do a screenshot right now. The option “disable backup “was greyed. Probably because the hard drive I backed up in my dropbox backup plan was disconnected. I can’t connect it anymore since it doesn’t exist anymore. I’ve tried with other drives to understand what happened with this one and why I can’t see it anymore in the app. During this process I added and disabled several drives, and it seems that a drive that has been disabled, can’t be added a second time. I tried to do a clean install of dropbox, and the external drive finally disappeared as expected. But I can’t add any drive since they do not show in dropbox backup app. I tried to unplug all my drives unlog dropbox app, log again dropbox, And plug my drives again, and none of my several drives appear. These drives are APFS encrypted or HFS+ And none appear