I am running a Pixelmon 3.5.1 on a 1.7.10 Cauldron server using version Quests 2.0.0 ALPHA. Citizens NPCs do not work on my server so I was wondering how can I add a quest to a sign if possible? If not, do you know a plugin that is 1.7.10 Cauldron compatible that can do this? I really want to have quests for players on my Pixelmon server.

That version of Quests is not supported. Cauldron is not supported either, so if the latest version available on Spigot does not work, try an older version.

A quick Google search for "minecraft command signs" should yield helpful results.