What to do if I was told by my doctor that I was not allowed to lift push or pull anything over 20 lbs but my boss insists that there is no “light duty”?

If your medical condition qualifies as a "disability"--which it may; a disability is something which has a major imact on ordinary life functions (like what you can lift or move)--your employer must make "reasonable accomodations" to it under the law. However, a reasonable accomodation is a change in how the job is done, or the provision of some assistive technology or device, which will allow you to do the job you were hired to do. That is the critical point: if you simply cannot due your job, then the employer may terminate you--the employer is not required to transfer you to a different position, to make up a new job for you, or to retain and pay you when you can't do what you were hired to do.

So if the truly is no "light duty" available for your position--that is, if the restriction means you can't do your job--you employer may terminate you, or take any step short of termination, like suspending you, demoting you, putting you on furlough, etc.

Only if you have a job that could meaningfully do while not lifting, pushing, or pulling more than 20 lbs--that is, you can do your job with that restriction, without costing the employer too much in terms of money, efficiency, or disruption--would the employer possibly be obligated to give you light duty.

Remember: you doctor is not an executive of your company, and has no direct authority over it.