Where are your backups located? What options did you select when creating the bootable USB stick? Backups are not moved to the USB, so the storage accounts need to be available for the restore to take place.

OK, I assumed it was like Windows recovery which puts image on USB stick. My backups are on synology nas. I enter my storage credentials and it properly displays with green check and "Restore only" indicator. However, it then throws a dialog saying "Cannot find any prefix in this backup storage". What do I do now?

on the computer where you're creating the USB, does it have the same prefix as the computer that you're looking to restore?

Yes, same pc. I have to go out for rest of day but will get back to this tonight.

8d suggest you open a support case to get this answered.

How best to manage that without a working PC? I am am trying to recover my only PC. Can we do support over phone?

You can open a case from here on mobile if necessary. https://support.msp360.com/