Explanation:

The encoded message was:
T I K T R I E P O I H N I T Y T X L R T (There are 20 letters)
We know that the letters were divided into pairs in the first stage of the encoding. The puzzle is to find out what happened next.

How about dividing the 20 letter string into two 10 letter strings like this:

T I K T R I E P O I
H N I T Y T X L R T

Hmm! Well, what will happen if I take a letter sequentially from each row starting with T in the top row, then H in the second row, then I in the top row and N in the second row – and so forth like this

T H I N K . . . . . I think we are getting somewhere!

Explanation:

The encoded message was:
E R T T R E P A N Y E O E N I C C P D P V T R Y (24 letters)
If you tried it first as if it had been encoded using a paired grouping of letters you will know that did not work. So, now try it as if the letters have been grouped in threes. There are 24 letters so we will divide the long string into 3 rows of 8 letters each

E R T T R E P A
N Y E O E N I C
C P D P V T R Y

Now we will take the first letter from each row, then the second letter, then the third – and so on like this:

E N C R Y P T E D T O . .etc. . . . Aha!