Many have issues setting up a mission so here is a basic one I use to test usually. Note that for me it does not work on creative server, only adventure. I have circled red the items I entered. the item id is my mission ruler, so when I examine it, it spawns 1 acorn in my inventory. The mission will repeat over and over as many times as you want to examine. (you create missions with the 'mission ruler' you can spawn one with an ebony wand)
In this screenshot there is an 'x' in
There is a bug (or feature that i don't understand) where RULER mission triggers are checking that a matching EPIC mission with the same ID is active.
On the default Creative database, the tables for both types of missions start from 1, causing their ID's to collide and break things. This happens even if you have epic disabled (some default inactive missions are generated anyway).
The fix i found is:
Backup your stuff
Create a dummy mission, doesn't matter