How to find Fortnite letters?

I know that at Seaso 11 in Fortnite you can find the letters F-O-R-T-N-I-T-E to accomplish missions. I went to the letter N, but there was no letter. I heard on youtube that you have to do certain tasks first. Do any of you know which tasks? And if that is not correct with the tasks, what do you have to do?

I think you have to solve the previous missions first when the task is shown with the letter it also appears on the map

OK thanks