Is this correct?
 

Did I say this correctly: "Paga lo que debes. Esa es la regla indispensable"? Does it sound unatural or something? Thank you.

"Indispensable" is not the right choice there

In Spanish it's used more in the sense of irreplaceable, which the English word hasn't.

You may use in Spanish "esencial" in the meaning you seem to need there (absolutely necessary), or replace it with "inexcusable" (in Spanish: that cannot be avoided using pretexts or excuses)

Thank you :)