Download application

See the following Apple articles:

If you can’t download or update apps on your iPhone or iPad

https://support.apple.com/en-gb/HT207165

Require a password for App Store and iTunes purchases

https://support.apple.com/en-us/HT204030

You may need to have a payment method on file, even if the app you want to download is free. Check which payment method you have on file