Frank Cross is producing a live televised version of A Christmas Carol, yet he never seems to recognize the events of Dickens' story are happening to him in real life. He even quotes the story while visiting Claire at the homeless shelter, so he's clearly familiar with it, but when the Ghost of Christmas Future shows him the casket, he doesn't know who's inside there. What gives?
I think he does recognise what's going on, but imagine if you were subjected to the events of a story you're televising - I think we'd all struggle with the rather convenient irony and timing of it. Frank seems to accept what's going on by the time the third Ghost of Christmas arrives, too, but I think because the Ghost shows him his assistant's son, then Claire - both of whom have basically suffered and changed because of him, he initially assumes his brother's in the casket, and then... Well I guess he struggles to accept at first that even his level of jerkassery means he'd die alone with no-one really caring to attend his cremation but his brother and sister in law. The Ghost then goes the extra mile by putting him inside the casket as it burns, to make the point; Whereas before, Frank was still being cynical and slightly wisecracky and claiming that he could solve things - most likely assuming that they could be solved with money. Putting him inside the casket tells him he's going to burn, as he emphasises when taking over the broadcast. That his level of jerkassery will leave him alone and dead and miserable with no-one but his ever-reliable brother; So I think it's initially disbelief. Certainly, he may have expected to see something hokey, like his own tombstone, but.. Yeah. I think he's in disbelief that his attitude may lead to his lonely death, at first.
He just didn't realize it because the first thing he saw was his sister-in-law. Presumably he didn't think she was close enough to him to come to his cremation, he he just assumed the casket was his brother's (which on it's own would've been plenty depressing for him). It wasn't until his brother walked into the room that it clicked, leading to the knowing question, "Who's in there?"