I'm not sure why is not an acceptable way to express struggle. From Le grand Robert:
© 2023 Dictionnaires Le Robert - Le Grand Robert de la langue française
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.
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