He didn't know how to do it. He promised Anakin to stop Padme's death. But once Anakin turned darkside and wanted to collect on the promise, Palpatine immediately says he can't do it, but together they can figure it. I think it is a line to keep the apprentice in line. If the apprentice eventually wants to kill the master, the best way for the master to stay alive is for the apprentice to think there is "something more" to learn. Once they learned it all, the master is no longer needed. So if the master has the apprentice working with him on something immpossible, they are more likely to put off killing the master. Maybe Palpatine thinks its possible, because his master told him/tricked him into thinking he could stop death. But more likely, he knows its a lie, probably one he fell for, and he is just passing it along because he needs to keep stringing Anakin along.