My Previous employer is not issuing offer letter.

[batman99]

hello all,

I am an OPT student and my opt ends at June 28 2018. I need to apply for stem extension. For that I need offer letters from all my previous employers. But my previous employer is not issuing the offer letter.He started his company on august 2017 and I joined there. I left that employer by intimating them before 1 week because of some problems with them. From then my relationship with the employer is not good. Now he is delaying to issue my offer letter. I am just asking him to issue a letter for the period I stayed there which is from August 30 2017 to October 23 2017. Now he is not lifting my phone calls and not responding to my messages and emails. My question is, is it required for an employer to give a letter to his previous employee even if he left that company until the period he worked?? If he doesn't issue the offer letter Can I take any legal action on that employer? Does this offer letter effects my stem opt application??

Thanks in advance