Says that I already have it installed (whatever that means). I’m running 10.13.6 High Sierra
This problem happened all of a sudden. Up until yesterday it was working just fine 
Go (top of the desktop) > Go To Folder. Paste this line in the box, or type it in. Do Not leave out the Tilde (squiggly line) at the front. Up-Left on the Mac keyboard.
~/Library/Application Support/audacity
Delete everything you find in the Session Data folder (but not the folder itself).
Was that folder blank?
Koz