Apple has stopped signing iOS 13.6.1, meaning that it is no longer possible to turn back to the earlier version if you have already upgraded to iOS 13.7.
Apple typically stops signing code for previous software versions soon after releasing their successor. That’s because every software update Apple creates is supposed to be faster, more stable, and — most importantly — more secure than the previous version.
It’s therefore in the best interest of users to stop them turning back.