The military is exempt from the ADA (American's with Disabilities Act). This is the federal law that prohibits discrimination based on disability.
So while generally speaking, an employer must make reasonable accommodations for a person with a disability, these rules do not apply to military service.
So if you have a disability that could, potentially, limit your ability to perform your duties (whether deployed or stateside)? That can be a basis for the Army (or the Guard) to refuse to give you job.
Since the military is exempt from the ADA, they can refuse to provide the job based on your mental health history.
Now...you raise a good question. If you are not capable of handling the stateside job...why are they deploying you??
That is, unfortunately, a complete separate issue.
Your commander has the authority to order you to deploy...this is inherent in their role as commander. So they can order you to deploy...even if you have a disability.
That said, you make a valid point...if you have a disability that would preclude you from this particular job, then it makes not much sense that they are not running you for a MEB (the medical evaluation board...the process used to separate a soldier not physically qualified for their job).
So you can request that your commander run you for a MEB (to determine if, in fact, you are not able to perform your duties). If you have PTSD that limits your ability to serve I would expect the MEB to recommend you be retired.
Please let me know if you have more questions. I am happy to help if I can. Otherwise, please rate the answer so I may get credit for my work.