Preinstalled Apple Apps

• Settings > [Tap your Name] > iCloud > Photos > Optimise iPhone Storage (Refer Pic 1}
• Settings > iTunes & App Store > Offload Unused Apps [Enable] (Refer Pic 2}

Built-in Apps (Some of them can be deleted)

• Camera
• Photos
• Health
• Messages
• Phone
• FaceTime
• Mail
• Music
• Wallet
• Safari
• Maps
• Siri
• Calendar
• iTunes Store
• App Store
• Notes
• News
• Contacts
• Books
• Home
• Weather
• Reminders
• Clock
• TV
• Stocks
• Calculator
• Voice Memos
• Compass
• Podcasts
• Watch
• Tips
• Find My
• Settings
• Files
• Measure
• Magnifier
• Shortcuts
• Translate

Try these four options. Try downloading after each step

1. If you have a VPN enabled, disable it.
2. Try restarting your iOS device
3. Enable AirPlane Mode and Disable
4. Reset Network Settings (Settings > General > Reset > Reset Network Settings.)
5.