Hello @Aishwarya Upadhye Thank you for your post and apologies for the inconvenience this must have caused!
I see that you're trying to backup a VM on Azure stack HCI using MABS and being greeted with error message. There is a known issue with older agents. There was a change on AAD Authentication which was fixed on newest agent. So as work around download latest MARS Agent and replace the Installation File of old MARS on the extracted installation MABS under MARSInstaller folder.
Hope this helps!
----------------------------------------------------------------------------------------------------------------------
If the response helped, do "Accept Answer" and up-vote it