It’s possible that the Thai clerk that is processing your visa application got the impression somehow that you’re traveling for academic purposes. Is this the case? If it is, it’s possible that this letter is required, and if you can’t get such a letter, you will have to change your plans to exclude Thailand.
If your travel is strictly personal and has nothing to do with your education, make sure that your visa application states this, and try your application again.
Credit:stackoverflow.com‘