"I just can't" may have been around forever, but it's certainly current and somewhat idiomatic. In French, that would be "Je ne peux tout simplement pas," more words than simply "Je ne peux pas." When Duo states it simply, people want to muck it up, complicate it, add, subtract, or propose something different. Predictable; never fails.
It’s best not to compare French grammar with English grammar because they are very different. It’s also not ideal to translate word for word.
The “ne” and the “pas” work in tandem to make the verb into a “not” verb.
- I am walking = je marche
- I am not walking = je ne marche pas
- you are = tu es
- you are not = tu n’es pas