They can't force you to resign. But they can terminate you any time they want, for any reason whatsoever, unless you have a written employment contract protecting your employment. Therefore, this might better be considered "resign, or else"--i.e. resign, or be terminated/fired. Of course, if you resign, you will not be able to collect unemployment insurance/benefits, whereas if you are terminated, as long as you were not fired "for cause" e.g. for insubordination, violating company policy, etc. you may be able to receive unemployment therefore, you may wish to not resign and let them terminate you.