Compression of a text that contains only the 5 first letters of the alphabet?
If you look at the algorithm for creating Huffman trees/codes, you will see that while it uses frequencies, any weight works. You can always rescale (by dividing by the sum of all weights), if you wish.

If your problem does not contain any weights but only a relative order, you can not build a Huffman tree; you don't know where in the order the inner nodes you create go. Ask your teacher.