都可以。把bank看作一个点,就用at, she is at the bank.把bank看作一个面(较大的一个范围),就用in, she is in the bank.
in the bank
at; if you want to emphasize she is inside the bank, you may use in.
in the bank 因为银行是室内,at有点缚在表面的意思。例如:鸟在树林里 the bird in the tree 而不会说at the tree