I had the same error message.

In my case, I installed makemkv in Ubuntu by using snap (i.e. by using the Ubuntu Software Store).

To solve the problem, I went into the snap store, clicked on "Installed", scrolled down to the makemkv item, clicked on it, clicked on "Permissions", turned on both of the "Read from CD/DVD" buttons (there were two for me, probably because I have two DVD drives).

I also turned on "Read/write files on removable storage devices" although that may not have been needed.