This cipher has me vexed. FOOTOOLTLTLBDCLOLTLTJBFTOCFTOOOCDC. There appear to be too many Os(and no other vowels) to be a transposition cipher. The coindidence index is 14.75, which is quite high, but with the repeating LTLT and the triple O no substitutions have made sense.
Okay I think the message is
Don't trust the Kenku.
As you pointed out, it looks like the letters are actually broken into pairs rather than encrypted singly (see the frequency of the pair LT) so let's break the ciphered text up into pairs of letters
FO OT OO LT LT LB DC LO LT LT JB FT OC FT OO OC DC
This looks rather likes a Polybius square cipher where we are using letters instead of numbers. I played around a bit to try to determine the order of the letters on both axes and determined that a very nice thing happens if we use the order CABOT on the horizontal and FJOLD on the vertical axis (also Cabot Fjold sounds like it could be a place in DnD).
In particular, the Polybius square looks as follows
C A B O T
F A B C D E
J F G H I J
O K L M N O
L P Q R S T
D U V W X Y
To decipher, for any individual pair of letters, you take the first letter on the vertical bold axis and the second letter on the horizontal bold axis and find the letter in the grid with the corresponding coordinates, so for example LT becomes T.
Translating, our ciphered text becomes
DONTTRUSTTHEKENKU = DON'T TRUST THE KENKU
This is backed up by the fact that a Kenku is a character from Dungeons & Dragons.
I've just realised "Cabot" and "Fjold" are mentioned in the comments so this must be the answer.