If a company filed Chapter 11 8 months ago,are they legally bound to pay in the agreed 30 days the $20000 of credit I gave them?

What was the money used for?
If your claim has the status of an administrative expense, it is first priority to be paid.
I take it they have declined to pay you, so sounds like you need to go to court.
If the money were used for necessary operating expenses of the business, it will be an administrative expense claim.
I would check that with a lawyer, then you can file the claim, get the form from the court clerk, should be available online.
Send that claim to the attorney for the debtor company.
Make the guy you dealt with get you a copy of the bankruptcy notice.