There are two versions of Garnett that correspond with the two versions of Monocle (monocle 2 and monocle 3). If you go to the docs website here:
https://cole-trapnell-lab.github.io/garnett/ you'll see at the top 'Documentation' and 'Documentation for Monocle 3'. If you're loading monocle using library(monocle) then follow the install docs under 'Documentation', and if you're loading monocle using library(monocle3), follow the install instructions under 'Documentation for Monocle 3'. That should solve the issue.