You should have received an invitation at level 45 - have a look in your inventory. When you find it click on it and it will give a quest to go to undaunted enclave.
In any case, perhaps the reason is because you're not part of the undaunted at all. Have you talked to the NPC at the starting city? Have they given you the little quest to check out the local Group Dungeon? If not, then that's probably the reason you can't do pledges.
If I recall correctly, this happened to one of my characters and I fixed it by following these steps:
- Go to the entrance (outside, in the world) of the Group Dungeon in the first zone according to your alliance.
- (Fungal Grotto, Stonefalls -- if EP)
- (Banished Cells, Auridon -- if AD)
- (Spindleclutch, Glenumbra -- if DC)
- Just outside the door, you should find your local Undaunted homie, double dog daring you to go inside to prove you're tough, and not some soft momma's boy. All you have to do is walk in the dungeon, then port out. Joke's on them, heh...
- After that, go to the main city in that same first zone (Davon's Watch, Vulkhel Guard, or Daggerfall) and find the tavern. It's the mug/stein/tankard looking icon on the map. Inside you'll see the one who challenged you to enter the dungeon. Talk to them, have the mates sing you a cringy song, then...
- ???
- Congrats, you're officially Undaunted, and can go meet & greet with the Pledge givers at the Enclave. From there on you're able to do pledges and whatnot normally.