Why I cannot split my iPad keyboard on iPadOS 14

That is no longer possible.

The replacement for this is the smaller, mini keyboard which you invoke by taking your thumb and forefinger and do a pinch inward gesture atop the iOS virtual keyboard.

That mini keyboard can be placed anywhere on the iPad’s screen.

To return to the full keyboard, use a pinch outward gesture, with your thumb and forefinger atop the virtual mini keyboard.

If you want the split keyboard to return you can leave feedback with Apple in the link below.

Feedback - iPad - Apple

Best of Luck to You!

The Split Keyboard is not supported by any iPad with a screen size greater than 10.5”. As such, all 11” and 12.9” models of iPad Pro - and the new fourth generation 10.9” iPad Air - do not support the Split Keyboard. These models support the new Floating Keyboard in addition to the standard Full-size on-screen keyboard.

All other models of iPad (including the 9.7” and 10.5” iPad Pro) - running iPadOS 14.x - continue to support the Full Keyboard, Split Keyboard and the new [small] Floating Keyboard.

Use the floating keyboard on your iPad – Apple Support

Split and move the keyboard on your iPad – Apple Support