take a screen shot on ipad without using volume buttons

If you have an Apple Pencil, a further option exists to trigger a screenshot...

Using the Pencil with iPadOS14.x, from the bottom-left or bottom-right corner of the screen, swipe toward the centre of the screen; this triggers a screenshot.

For iPadOS13, you can swipe from the bottom-left corner of the screen toward the centre. Swiping from the bottom-right corner is not supported.

Without a Pencil, the only means to trigger a screenshot from the iPad are the physical buttons.

Take a screenshot on your iPad - Apple Support

Use AssistiveTouch on iPad - Apple Support may offer some options.