"When had you entered the museum?"
Translation:Quando eri entrata nel museo?
I used the same website and got it wrong. Who is right and who is wrong then? The website or Duolingo?
Entrare takes "essere" like all the verbs that show a translation: "sono entrato", "sono passato", "sono tornato", "sono andato". ... "if you use "avere" then the only meaning could be "ho entrato qualcosa" / "I put inside something"
says entrare takes avere. is that wrong?
As written above in another comment, it can take both, depending on meaning. If you enter somewhere, it's an intransitive verb of moving and takes essere. If you enter (put in) something somewhere, it's a transitive verb and takes avere.
Can someone please explain why can't it be 'in' museo? In other words, does it have to be 'nel' museo?
The simple answer is that we're talking about entering the museum, and in + il = nel.
The more complicated answer involves the use of the definite article in Italian, and how it's different from English. If you were talking about going to an unspecified museum, you might say in museo, but if you're asking a fellow museum-goer at the Museum of X when they crossed the turnstiles to enter said museum, you'd say nel.