The one time pad can be cryptographically secure, but not when you know the key. Can you solve this? We've given you the encrypted flag, key, and a table to help UFJKXQZQUNB with the key of SOLVECRYPT
The table is simple - the you grab the plaintext character and the corresponding character from the key and cross-reference them to find the ciphertext character. To reverse it, you find the key character and go along the row (or column) until you find the ciphertext character, then you go perpendicular to it to find the corresponding plaintext character. This nets you CRYPTOISFUN, so the flag is picoCTF{CRYPTOISFUN}.
This is actually a Vigenère cipher, so you could also use an online tool to do it for you!