Hello, thank you for the question.
It's clear that the company is saying to you that you have been fired "for cause" and that you're entitled to any severance.
Being fired "for cause" is normal when the employee has stolen from the employer, or repeatedly violated the rules of the workplace, or has assaulted a co-worker, or something serious. If you don't know the reasons why the company has fired you, then you should ask in writing and ask for a written reply. They don't have to write back to you, but you should still ask.
If you belong to a trade union, then obviously you speak to your union steward or representative about it and they'll take action on your behalf.
If you don't get any information from your employer about why they fired you, then go see a civil lawyer or employment lawyer near you, and have the lawyer write the company a letter about it. That shouldn't cost much. You've been there for 20 years, so if there's a chance of getting a severance package you should try.
Remember that it's not true that a severance package is due to the employee in every firing or layoff. Even if they should be giving you a severance, they obviously don't want to and so you have to put some effort into making them.
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