In case this helps anyone, I had this same error today. PI was working fine, tried to upgrade, upgrade stalled, PI then wouldn’t boot. So tried to reflash the SD card to restore it to working order.
Used the OSMC installer, followed the instructions, got this error.
Tried a different SD card, worked perfectly.
Turns out the original SD card had some failed sectors. Didn’t show up during the write of data to the card. Only symptom was this “couldn’t mount bootfs”.
If you get this, try a different SD card. When an SD card dies, it’s isn’t necessarily immediately obvious that it’s the SD card itself. (It was a Kingston card that died).