Playfair Cipher Confusion with one column encryption
TR would be replaced using rule 2 of the Wikipedia description:


  If the letters appear on the same row of your table, replace them with the letters to their immediate right respectively (wrapping around to the left side of the row if a letter in the original pair was on the right side of the row).


So TR would become PS. See also step 10 of the example there.

Note that T and R are on the same row, not the same column.

You may want to use a larger key (i.e. one that includes more characters of the alphabet).