Ahmad, where is the letter draft that you wrote? We have nothing to review,advice, and help you correct the content of your work if you have not done the work yet. I hope that you do not think that we will do the letter writing for you here. We only advice you regarding your work so we can help you become a better writer. We are not a writing service. The instructions of your teacher are very clear and does not require any additional explanation. However, if a part of the instruction is unclear to you, then ask for a clarification here and we will do our best to explain it to you.
Just be imaginative and creative in writing the letter so that you can complete it. Imagine the kind of company you hope to work for and what position that is. Then think of a situation that will prevent you from starting on your dream job. Those are the two starting points that you can use for writing the draft of your letter. The rest of the information should flow naturally for you from those points.
Again, if you need additional advice, then post your question. But don't ask for an example about how to write it. We will not do the writing for you. You won't learn how to write better in English that way.