In what state is this?
Experience letters are a rarity in the US. Many employers will not have heard of them. In most states employers are not required to provide them at all, and in the few states that do require them you only have a limited time to make the request and you may be required to make it in writing. If you miss the window or do not make the request using the required format, the employer may ignore your request.
If you can provide your state, I can at least tell you what, if any, requirements there are on the employer to provide you with an experience letter.

