I have a stand-alone app (with no companion app on the iPhone) and one of my users had the same message: "[App] Needs to Be Updated — The developer of this app needs to update it to work with watchOS 27." After reading above solution I asked the user to remove the app and re-install the app. This worked perfectly fine: the watch apparently loaded the 64-bit version of my app and the app worked fine, with no error messages. I agree. The error message is wrong: Apple should not refer to the developer to update the app, but Apple should install the correct 64-bit version after install of WatchOS 27.