avoir du mal a

Question

I'm not sure why is not an acceptable way to express struggle. From Le grand Robert:

II V. intr. (Fin XIe). Se donner de la peine, du mal*. ➙ Appliquer (s'), efforcer (s'), évertuer (s'), fatiguer (et se fatiguer), gémir (fig.), trimer (fam.) ;→ 1. Coût, cit. 27 ; élite, cit. 5. Peiner comme une bête (cit. 11) de somme, comme un forçat… Peiner jour (cit. 42) après jour. — Élève qui peine sur une composition (cit. 8) latine (au sens III, 2, de peine). Il peinait pour s'exprimer (→ Facilement, cit. 3). — Par ext. Respiration, souffle qui peine (→ Oxygène, cit. 3).

Accepted Answer

Max, I think some added context might help - can you elucidate further ? 
You can certainly use  “avoir du mal à/avec .. “ in some contexts that can be interpreted as ‘struggle..’ in English, but it is not necessarily the appropriate French expression to translate all English uses of the word ‘ struggle ‘. That is covered in the lesson. 
There are also nuanced but meaningful differences in the use of words such as struggle, fight, combat, battle etc in English. 
I don’t see anything in Le Grand Robert citation that would indicate that the lesson is incorrect. There are several senses of the noun ‘mal’ and many expressions - Larousse link.
If you can provide more information and examples it will be easier for others to comment further. 
https://www.larousse.fr/dictionnaires/francais/mal/48790
https://www.wordreference.com/fren/avoir%20du%20mal