Hello Yang! i agree with some of the points mentioned before. But I would like to add the following: I think that you may be confusing "grasp onto something" (which means to physically hold something that is why the preposition "onto" is used - to explain that the person is physically holding the object in their hands) with "grasp something" (which means to fully understand something). So the correct way of writing it would be: "She can’t grasp the answer". If the person is unable to grasp a specific answer then the correct determiner would be "the" and not "an"- "an" would be used if the speaker was talking about asking any random question so "the answer" refers to a specific answer and "an answer" would refer to any random answer. I hope that helps!

Sounds too funky. I’d recommend “She can’t grasp the/an answer.”

Thank you Alan and Olasubomi!!!

An answer or the answer? In any case, that would be an unusual expression. To grasp "a concept" is a common expression.

she can't grasp the answer. you can't conjugate two verbs in a sentence