I worked for a company for 16.5 years. I never really got support from upper management. There was a Union I felt I had no other choice but to sign a last chance agreement but still feel I should get severance pay. Do you think I have a case

Because you are in the union, you must go through them to fight for your severance pay. If you were not in a union, you would be entitled to severance pay unless the employer could establish just cause. To do so, the employer would need to show that the misconduct was very serious or you had been warned many times and refused to correct it.