I've walked to several locations so I can fast travel and it says "Fast travel is currently unavailable from this location"

This is really annoying so please help, and if this has anything to do with starting the "Eureka" quest, then I'll just revert to my last save.

Are you sure you are not in a building? You can only fast travel if you are outside you know.

Make sure your not encumbered.
There are many places you can't fast travel to or from until you're outside the building or area just go out of the area 10 feet
and then mark and fast travel from there.
This drove me crazy until The first time I tried fast travel while in the fort and when I got out of the building I tried again and no problem.

Trust me, I tried everything.
I have the Long Haul perk so I know I can fast travel while over encumbered.
I know for a fact each time I tried to fast travel, I was not in a area or in a building.
I had started the Eurka Quest, then made my way out of the Hoover Dam, then fast traveled to Cotton Cove, that's where I freed some people, then couldn't fast travel ANYWHERE,

it's okay, though I ended up reverting to my last save, thanks anyways.

What I bolded is where you went wrong.

How did I miss that? The game specifically tells you in the popup right before the quest.

It has to do with the Eureka! or You'll Know When It Happens quests.

Though, I was able to fast travel out to Novac from the Ranger Station close to the Dam (forget the name), and then fast travel back to the Dam. I think.