Unknownreported this bug
Presently there is the start of untested builds for 1.17 in the git, but the builds are not compatible. I've tested it and get the following error:
Tested Versions: Paper Spigot 1.17.1 (in house modified build of 1.17.1-R0.1-SNAPSHOT) MCCosmetics Version(s): 3.0 and 4.0 MythicMobs Version(s): 4.13.1/5.0.2/5.0.3 ModelEngine Version(s): r2.2.0/R2.5.1