Hello!
I do not know why but it looks like some people having problems with the Quests addon. Anyway this one work with with mm 2.5.1 and spigot 1.8 - 1.10.2 and should work with mm 2.6 and 1.11 too but its not tested there. For example look at the github readme or use the quests ingame editor.
https://github.com/BerndiVader/questsKillMythicMobsObjective
I do not know why but it looks like some people having problems with the Quests addon. Anyway this one work with with mm 2.5.1 and spigot 1.8 - 1.10.2 and should work with mm 2.6 and 1.11 too but its not tested there. For example look at the github readme or use the quests ingame editor.
https://github.com/BerndiVader/questsKillMythicMobsObjective