An idiom.
"tratar de" means "to deal with", so it is a literal translation
Why not "I can deal with that"?
It should also be accepted.
Does "I can manage that" also correct? If not, how do you say that in Portuguese?
I think it should be accepted also.
But still isn't!
How about "I can take care of this" which can have the same meaning as I can handle this? Not accepted.