Apparently, the company considers you less than acceptable in your attitude with others. You cannot sue them for wrongful termination because CA is an "at will" state which means they can hire and fire at will without giving any reasons.
I suggest you take the course they want you to and avoid being fired.
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Unless your employee manual provides for progressive discipline, you can be fired without notice and for no reason at all. While you may think you are well-liked, obviously some people feel differently. You need to find out specifically what problems they have with your attitude and address them. As my colleague said, CA is an at-will employment state and you have no claim for wrongful termination on the facts you provided.
The response given is not intended to create, nor does it create an ongoing duty to respond to questions. The response does not form an attorney-client relationship, nor is it intended to be anything other than the educated opinion of the author. It should not be relied upon as legal advice. The response given is based upon the limited facts provided by the person asking the question. To the extent additional or different facts exist, the response might possibly change.
If you have no written employment contract and no union, your employment is "at will," and you can fired without notice for any reason or no reason, and you can quit on the same terms. The company has to follow its own policies incuding those regarding warning/discipline/termination, but it can fire you for less than what it sees as an attitude problem.
Many people seem to be underr the mistaken impression that they have a right to keep their jobs, barring some serious misfeasance and malfeasance. In France, for example, all workers have job protections and can't be fired as "at will" workers. But this country is different. If you want that to change and you want more pro-employee, anti-employer laws, always vote, and always vote for the most progressive, liberal Green party candidate you can.
Disclaimer: Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.