Deceased Parents with no Will

My wife's parents passed in November of 2023.  There is no Will, no executor or personal representative.  I'm trying to file their 2023 return but it appears I'm not allowed to file until an executor or a personal representative is assigned.  Is it true I'm not able to file their 2023 return or file an extension before an executor or a personal representative is assigned?  They are due a refund.