How to set automatically login
Can’t login automatically in App Store
Question marked as
What do you mean you can't log in automatically?
You should not need to log in to the App Store on an iPhone at all.
If you have your Apple ID set in Settings->Apple ID->Media & Purchases, it should keep you logged in to the Apple Store always.
If you mean it is requiring a password for purchases, then you can configure that in Settings->Face ID & Password->iTunes & App Store to use Touch ID or Face ID for purchases.
You can also set the time between requests for your password or Touch ID / Face ID in Settings -> Apple ID -> Media & Purchases -> Password Settings.
click here -> Require a password for App Store and iTunes purchases - Apple Support