How to compatible iPad to download app

That depends entirely on the type of iPad and which iPadOS is installed.

Most apps require iPadOS 13 or 14 or later.

See -> Identify your iPad model

Identify your iPad model - Apple Support

If your iPad is eligible for an update, then go to: Settings -> General -> Software Update

You can check by reading this Apple article:

iPad models compatible with iPadOS 13 to iOS 15.6.1

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

You can update your iPhone to iOS 15, see this Apple article:

iPhone models compatible with iOS 15.5 – Apple Support (UK)

Go to Settings -> General -> Software update.

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

You need good Wi-Fi.

If they are On, turn Off Hotspot & VPN.