As Petra says, you can read this in the manual.

https://zorro-trader.com/manual/en/ta.htm

You must use the return value of your existing ADX call, rather than dereferencing a function as an array (very illegal).

Do you know how functions work? Read here:
https://zorro-project.com/manual/en/function.htm

Do you know how vars and series work? Read here:
https://zorro-project.com/manual/en/series.htm
https://zorro-project.com/manual/en/apointer.htm

First you had only wrong number of parameters, but then your code became totally wrong.

Correct is ADX(18), not ADX[0]. But do not copy-paste it but learn to write correct code.