The challenge is the following,
We are also given the file message.txt which contains,
Ta _7N6DE7hlg:W3D_H3C31N__BD4ef sHR053F38N43D47 i33___NC
The challenge tells us that this was encrypted using the rail fence with 4 rails, so I went to Dcode.fr Rail Fence Cipher and decrypted it using 4 rails.
Decoding this gave me,
The·flag·is:·WH3R3_D035_7H3_F3NC3_8361N_4ND_3ND_EB4C7D74
Therefore, the flag is,
picoCTF{WH3R3_D035_7H3_F3NC3_8361N_4ND_3ND_EB4C7D74}