1) Download the MythicMobs folder here and put it at the plugins folder
https://www.dropbox.com/s/2eiepzrmfu8hq5a/MythicMobs.zip?dl=0

2) Use Paperspigot 1.17.1
https://papermc.io/api/v2/projects/paper/versions/1.17.1/builds/409/downloads/paper-1.17.1-409.jar

3) Download MythicMobs (Free)

4) Start the server

5) Place down item frames

6) Restart at least 2-3 times to see the effect.