Cabal version too high. Did you have a pre existing cabal installation independent of stack or something?
Unfortunately, the reason for this is that ghc-mod still isn’t updated for GHC 8.2. It’s almost there—a package candidate has been uploaded that works alright—but it hasn’t made it into the LTS yet. There’s a way to build and install the package candidate in a way that will work with stack, which I have done on my personal machine, but honestly I decided it was just too much for this (already overly long) blog post.