I cannot download any app in my iPhone 6

iPhone 6 is an outdated iPhone (9 september 2014), can’t be update to a higher iOS.

iOS 12.5.5 is last security updates for iPhone 6 -> released 23 september 2021.

It seems that the apps you can't download require a higher iOS version.

See also the following Apple articles:

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

If you can’t download or update apps on your iPhone or iPad – Apple Support (UK)

Require a password for App Store and iTunes purchases

Require a password for App Store and iTunes purchases - Apple Support

abhi ~ Apple offers several newer iPhone models which will run iOS 15:

iOS 15 and iPadOS 15 are compatible with these devices

...Found by searching Apple's Knowledge Base.

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