can't log into my app on phone
can't log into my app on my phone to download a new app
If you mean you can't connect to the App Sore, this may help:
If you can’t connect to the iTunes Store - Apple Support
If it is something else, can you be more specific? What message do you get when you try? Is your payment information on-file with Apple & is it up-to-date?
Use the App Store app on your iOS devices, Apple TV, or computer - Apple Support