listlocalsnapshots "unrecognized verb"

I'm running a MacBook Pro (Retina, 13-inch, Early 2015) that is using macOS Sierra version 10.12.6. The storage for it is running extremely low, as I only had 128GB (or 120 as the computer says) to start out with, and under "About this Mac" and storage it says that the "System" is using 105.79 GB of space.

After digging around on Google, I found that it is most likely that time machine backups are what is taking up all of that space. Time Machine backups are not useful to me so I attempted to list them in order to delete them using this command in terminal:

sudo tmutil listlocalsnapshots /

However after inputting the command and entering my password, it came up as "unrecognized verb".

Is there a different command I should be using to bring up the time machine backups?

Or perhaps is there a completely different culprit as to what may be taking up that much space?

I have resorted to using an external flash drive that has 128GB on it, but it does not stop this annoying message from popping up every two seconds:

And then when I hit manage, it looks about the same as the first image.

If anyone would like to point me in the right direction I would greatly appreciate that.