"My grandmother is at the post office."
Translation:Моя бабуся на пошті.
баба has several connotations, one of them is quite pejorative. It sounds rude to say that about one's own grandma. It has slang meanings too(wench, broad, softy, milksop..) and can be misunderstood.
And yet we do say баба to refer to our grandmas. Especially in family conversations For example, I was taught to refer to my grandmas баба [name] and to grndpas дід [name]
You can, it's correct.
But not accepted here, please only use "to be" to translate :)
(I can elaborate why if you want)