He I can’t download barclays online banking

You need to have a good Service & Wi-Fi.

If they are On, turn Off Hotspot & VPN.

Charge the battery > 70%.

Then see -> https://www.barclays.co.uk/ways-to-bank/mobile-banking-app/

You can download from thw App Store -> https://apps.apple.com/gb/app/barclays/id536248734

Check also ⤵️

Manage App Store purchases, subscriptions, settings, and restrictions on iPhone

Manage App Store purchases, subscriptions, settings, and restrictions on iPhone – Apple Support (UK)

Prevent in-app purchases from the App Store

Prevent in-app purchases from the App Store – Apple Support (UK)