Wrong expansion. You have to run the dungeon that matches the rep you need.
ETA Sorry, I see this got corrected later.
OP, also hang on to all your Cenarion plant parts. You’re going to be wanting a boatload of them. Ditto sanguine hibiscus, and the aldor or scryer rep items. I’d recommend doing each dungeon at least twice as you level. It really helps to know them and with the rep later on.