How do I permanently disable the multi tasking side bar on my iPad Air?

Presumably your iPad is running iPadOS15.

Unlike iPadOS14 and earlier versions, multi-tasking features cannot be disabled from settings. Multi-tasking has received a significant overhaul, both simplifying its use and introducing new controls.

Key to accessing and controlling multi-tasking is the small “three-dot” button that is found at the top-centre edge of the screen.

More information about using multi-tasking can be found in this support page:

Use multitasking on your iPad - Apple Support

The iPad User Guide is also a rich source of information about using iPad and its many features. The Guide is available both online using a web browser (such as Safari), or as an Apple Books download:

iPad User Guide - Apple Support

https://books.apple.com/book/id1567104892

What multitasking side bar?

The Multitasking option bar is at the top of the screen.

Do you have an App on Slide Over mode?

Please explain your issue in detail. A screenshot may help.

Also see here: link-> Use multitasking on your iPad - Apple Support