at & in

‘My father had been working IN a bank last year.’

‘My father had been working AT a bank last year.’

From the above two sentences which one is right? I mean for IN or AT?

Also explain IN & AT?

'My father had been working IN a bank last year.this sentence is correct.

My father was working in a bank last year.

@beeesneees Can u plz explain why should not use ‘at’ in the place of ‘in’?

You can use ‘at’ too, The point I was making is that only the simple past tense is needed (unless there is something unexplained because the sentence is provided without further context).
Your use of ‘had been’ suggests that there is more information. He had been working at a bank then something happened to change that.