Hello will123iam,
Thanks for reaching out to us for support. It's never fun when you're unable to complete a backup of your MacBook Air via Time Machine, and we are going to do everything we can to help you. When you have a free moment, can you please tell us when you first encountered this issue and if you have made any changes to the settings on your Mac? Also, can you please tell us all the troubleshooting steps you have taken so far in an attempt to resolve your issue so that we can avoid repeating them? When possible, please be specific when describing the steps you’ve taken and the outcome after each step. Lastly, please tell us the version of macOS you are currently running on your computer, and check out the links below for resources that can potentially help to resolve your issue.
If you can't back up or restore your Mac using Time Machine
Find out which macOS your Mac is using
Thanks for using the Apple Support Communities.
Cheers!