I tried to book Rome to Florence 6/6/12 but "could not be confirmed in Trenitalia system". What does this mean?

I tried to book Roma Termini to Florence SMN 0n 6/6/12 at 10:45 for 2 adults 2nd class but at checkout it said "could not be confirmed in Trenitalia system" and asked me to take it out of my cart. The same thing happened for Venezia SL to Roma Termini on 6/15/12 at 12:27. What does this mean? How can I book it?