Justice Dept. policy, based on both Nixon and Clinton era memorandums, is that a sitting president cannot be indicted. Robert Mueller, who works for the DOJ, was likely expected to honor that convention.
But that does not mean he cannot issue a report recommending indictment, or that an Attorney General could not make the decision to try.
And that does not mean that an indictment after Trump were to leave office couldn't happen either.
Also, Giuliani has in recent times had trouble discerning fact from fiction, so his claims could be made-up or mistaken.
Here's CNN's report:
Special counsel Robert Mueller's team has informed President Donald Trump's attorneys that they have concluded that they cannot indict a sitting president, according to the President's lawyer, Rudy Giuliani https://t.co/2smCGfuK4i pic.twitter.com/PMCkmC1rdQ
— CNN (@CNN) May 16, 2018