tmutil - Failed to delete local snapshot
I encountered this issue when trying to delete the local snapshot from the Terminal offered by my Mac. The thing is that it booted in "installer" mode and I had to chroot /Volumes/Macintosh HD to be able to run the tmutil commands. The delete however didn't work in this installer mode.

So I had to restart the computer holding Shift to start in safe mode. From there, I could start a Terminal and run the above commands to delete the empty snapshot.