Can I sue my employer for trying to make me resign because I am on restrictions?

It depends on what those restrictions are. To oversimplify a very complex topic, the employer's only obligation is to make "reasonable accommodations' to your medical restrictions. Reasonable accommodations are ones not too expensive or disruptive for them and which let you still do the core or important functions of your job. Examples of reasonable accommodations include:
* If a cashier normally stands, getting them a chair or stool and letting them sit if they can't stand that long;
* An ergonomic keyboard, an ergonomic chair or stand desk, voice-dictation software, or a magnifying computer screen for someone who uses a computer and has carpal tunnel syndrome, vision impairment, or some inability to sit for prolonged times;
* Letting diabetic employees have extra breaks for snacks; an employee with IBS have extra bathroom breaks;
* If the employee's main job is not to lift, move, carry, etc. heavy things, to let them out of having to occasionally do it (e.g. not having to change the water cooler, if that's not their actual job)
BUT you must still be able to do your core job, and your employer does not need to create a new job or position for you or transfer you. So a nurse who must be able to lift and support patients could not do her job if she is under lifting restrictions; similarly, a shipping department or delivery employee could not do his/her job with lifting restrictions. In these cases, the employer does *not* have to accommodate them, since accommodating them means they can't do their job.
Also, an employer does not need to reduce an employee's hours as an accommodation: a reasonable accommodation lets the employee do his/her full job, not a shorter version of it.
If you feel that the accommodations you need are reasonable but they are not being granted, contact the federal EEOC about possibly filing a complaint.