Hey @Jefl8, are you still having issues with this or you managed to sort it? If this pop-up is still appearing, can you share a screenshot of it?
Thanks for sharing, are you entering your phone number correctly? For example +44 7817354479. If not, go to: Messages app > 3 dots (top-right) > Settings > Chat settings > Toggle the first option off and on. After this, go to: Settings > Apps > Messages app > Storage > Clear cache. Please also make sure that your apps are up to date from Google Play Store > My Apps & Games > Update all apps, and that your software is updated: Settings > Software update > Download and install.