I created a native app on Mendix and created my data base on web on I'm having a problem with my sign-in page on the administrator page its not working at all plus I'm having a problem attaching a registration
Skip native and create a progressive web app (PWA) instead. This will save you the trouble of having to build the app and adding it to the Apple appstore and Google playstore.
Don’t start in a blank app, but start with a ‘Base application’, which has less stuff for you to have to create yourself.
And for your administrator-page login, can you give us more details about where you get stuck.