I have noticed this being mentioned in several indicators in https://zorro-project.com/manual/en/ta.htm

Functions internally create series and thus must be called in a fixed order in the script.

Could someone explain what this means? what is the "fixed order"?

For every run() call, series() must be called the same exact number of times every time. It cannot be changed by an if statement between run() calls.

As a rule of thumb, put all series() calls outside of all if statements.

Ahh got it. Thank you.

This is a much better explanation, it should be documented somewhere if not already.