Long 15 is only unlocked if you nuked the NCR. If you prevented the launch or only nuked the Legion, you won't be able to access it.
If it's just the game glitching though, your only option would be to make a bit of a sacrifice(possibly losing some perk slots permanently).
1: delete Lonesome Road 2: load the game where you intended to go to Long 15 3: save in that game 4: reinstall Lonesome Road
You won't even be de-leveled, but any level up perks you'd taken from LR will be permanently lost if you're at level 50 already(e.g Alertness, Tunnel Runner, Just Lucky to be Alive). The non-level up perks you lose will be attainable again, as will everything else such as challenges, ED-E upgrades, equipment, holotapes, and so on.