АНГЛ. Что значит "...her elevator didn't go to the top floor" ?

The phrase means that this person is not very smart.
If he brought a book form another library to this library his elevator doesn't go to the top floor.
То есть винтиков (клёпок, заклёпок) не хватает (недостаёт) в голове, тупой, тормозит, недалёкий.

Про "недалёкость" и тупость идиом полно:
smb. has a screw loose <somewhere>; smb. is not all there; smb. has a tile loose; smb. has got apartments to let; smb. has bats in smb.'s belfry; smb. is a bit off <one's>; smb. is off smb.'s base; smb. is a button short; smb. has lost a button; smb. doesn't get (have) smb.'s button on; smb. has a cylinder missing

Синонимичных фраз море:
If you say that someone is, for example, several cards short of a full deck or one sandwich short of a picnic (or one French fry short of a happy meal), you think they are stupid, foolish, or crazy (!!!).

one card short of a full deck, a few bricks short of a full load; the light's on but no one's at home; not playing with a full deck - не все дома, ненормальный, сумасшедший, крыша в пути

He's not the sharpest tool in the shed or he's not the brightest crayon in the box - means he is a total dumb-ass.
В мягком варианте - Он не самый умный или он звёзд с неба не хватает.

Есть и синонимичные фразы именно с "лифтом" с тем же значением:
Elevator doesn't go all the way to the top floor / penthouse / mezzanine.
Elevator goes all the way to the top but the door doesn't open.
Elevator is on the ground floor and he's pushing the Down button.
Elevator to the brain suite is out of order.

Лифт ее доме не поднимался на последний этаж.

Ее лифт не идет до верхнего этажа

ее лифт не поехал на последний этаж..

еe лифт не пошел на верхний этаж