if app not compatible with my ipad version, how can i get previous version that is?

The tags appended to your post suggest that you have an iPad mini4. The mini4 is capable of update to iPadOS 15.8.x; the mini4 fundamentally does not support iPadOS16 or later. iPadOS 18 is current.

While many App titles remain compatible with your iPad, if you want or need to access to Apps that require a more recent version of iPadOS than your iPad is capable, you have no alternative other than to replace your iPad with a much newer or current model iPad.

Many App Developers only support the current and perhaps one or two previous major versions of iPadOS - and, as supporting earlier versions may be technically and financially inviable, will remove earlier versions of their Apps from the Apple App Store. Once withdrawn, these versions cannot be downloaded or (re)installed.

For information, here is a list of the highest version of iOS supported by the older models of iPad (for simplicity - “generations” are shortened to a number) that are restricted in their ability to be updated:

32-bit devices (obsolete)

iPad1 - iOS 5.1.1

iPad2, iPad3 and iPad mini1 - iOS 9.3.5 (WiFi Only models), iOS 9.3.6 (WiFi+Cellular)

iPad4 - iOS 10.3.3 (WiFi Only), iOS 10.3.4 (WiFi+Cellular)

64-bit devices

iPad Air1*, iPad mini2 and iPad mini3* - iOS 12.5.7

iPad Air2, iPad mini4 - iPadOS 15.8.4

iPad5, iPad Pro (9.7"/12.9" 1st generation)* - iPadOS 16.7.11

iPad6, iPad Pro (10.5"/12.9" 2nd generation) - iPadOS 17.7.8

(*) obsolete

Newer models are all able to be updated to the current version - iPadOS 18.5

• iPad Pro 13" (M4)
• iPad Pro 12.9" (3rd generation and later)
• iPad Pro 11" (1st generation and later + M4)
• iPad Air (3rd generation and later + M2/M3)
• iPad (7th generation and later)
• iPad mini (5th generation and later + A17 Pro)

"Old" versions are only available if on the device.