Faire du mal (emotionally)

Nous n'avons jamais voulu vous faire de mal. Why is it faire de mal? I thought it was invariable and it was always faire du mal

(Still having problems posting the extra information)
So doing it this way - I hope someone sees it

Bonjour Dragana !

The reason faire du mal becomes faire de mal here is because the sentence is negative (nous ne voulons pas).

See also the following lesson:
https://progress.lawlessfrench.com/revision/grammar/the-partitive-article-in-negative-sentences

I hope that's helpful!
Bonne journée !