Dear @mrh12,
Welcome to the Wefugees Community and thank you for sharing your question with us.
The requirements for the permanent residence permit (Niederlassungserlaubnis) vary slightly depending on your personal conditions (type of current residence permit). Please see this website, for instance, for an overview of the general requirements (here published by the authorities in Berlin), this list for the requirements for recognised refugees and this page for family members of German citizens.
From my experience in Berlin, you are supposed to submit a written application (formless) and add the required documents and proofs. I doubt that they will give you an appointment if they can not recognise that you fulfil the requirements. Regarding the 21 months of work you are mentioning above, I can not really understand how you got to this requirement. If you check the links I provided, you can see that the immigration office is asking you to "furnish proof of having paid compulsory or voluntary contributions to the statutory German Pension Fund for at least 60 months, or of entitlement to comparable benefits by another insurance company". You also need to be able to cover your and your family's costs of living. However, this requirement does not apply to recognised refugees and family members of Germans. As far as I know, you will not be asked to have worked for a certain amount of time. Instead, the immigration office may want to see a positive "prognosis" - meaning that the chance is quite high that you will be also able to cover the living costs in the future.
I hope this helps and please don't hesitate to get back to us with any further questions.
All the best,
Meike