screen mirroring from Macbook air to ipad

I believe that MacBook Air is too old to support Sidecar, or to act as an AirPlay Receiver.

System requirements for Continuity features on Apple devices - Apple Support

Sidecar is the feature that would let you use you use an iPad as an extra screen for a Mac.

Use your iPad as a second display for your Mac - Apple Support

Unfortunately, I believe that it requires a newer MacBook Air than the one you have – and I don't think the built-in Apple Pencil support would do everything you wanted it to do. The "scribble on the screen" feature would likely require specific support from the presentation application.

You want to see the Mac's screen on the iPad? Or the iPad's screen on the Mac?

Not sure what the storage space on the Mac has to do with it, unless you are expecting to use our iPad as a Mac computer, in which case this is not possible.

You may be able to see the iPad's screen on the Mac by using Quicktime, and setting the camera to the iPad, not 100% sure it works on Sierra though.

Perhaps if storage space is an issue, consider an external hard drive, and moving files and data off of the computer and onto the external drive to free up space on the Mac.

So yo want to see your Mac's screen on the iPad and be able to change what is seen on the Mac from the iPad?

This is not possible, sorry.