Unable to download any app from App Store

iPad Air, iOS 12

Update first your iPad Air to iPadOS 15.7.6.

Your iPad is eligible for an update, check here -> iPad models compatible with iPadOS 13 - iPadOS 15

iPad models compatible with iPadOS 15.5 – Apple Support (UK)

You need about 6 GB device storage and good Wi-Fi.

If they are On, turn Off Hotspot & VPN.

Charge the battery > 70%.

Go to Settings -> General -> Software Update -> Download and Install.

See the following Apple articles:

Update your iPhone, iPad, or iPod touch

Update your iPhone, iPad, or iPod touch - Apple Support (IN)

If your iPhone, iPad or iPod touch won’t update

If your iPhone, iPad, or iPod touch won't update - Apple Support

You can do the update via PC, see this Apple article: Update software on iPhone, iPad or iPod touch in iTunes on PC

Update software on iPhone, iPad or iPod touch in iTunes on PC – Apple Support (UK)

If you have a Mac see this Apple article: Update your iPhone, iPad, or iPod touch with the Finder

Update your iPhone, iPad, or iPod touch with the Finder - Apple Support

Your iPad running iOS 12 won't update higher than 12.5.7. That's too old for many apps now being distributed which typically may only support the last 2 iOS versions. If you want to run apps requiring a newer version you will have to buy newer equipment. Note that some services such as YouTube and Facebook may still be available if you use a web browser.