How do you reach the petal behind the barrier?

The barrier stays up until you finish the main story at the level 50 story quest. No item, puzzle or detour opens it sooner, so if your story is behind, that petal waits until you catch up.

That makes it the last of the eight map petals most players pick up. The other seven are open, so collect those first. The event menu marks every petal point, and the gathering petal task shows a photo of each location. Use the photo to confirm you are at the right spot, and to check whether this is the walled-off one before you spend time looking for a way in.

Being stuck at one petal does not stall your total. Petals keep coming in from first clears of each Alpha, silver collection badges, Elite Pathfinder Challenges, weekly rotations and the shop. The Legendary Journey event screen has a 'How to obtain' section listing the sources you have left, so check there while you push the story toward level 50.