Luke didn't have the slightest idea how to handle Sith Lightning at this time. He would be like anakin in AOTC. (always made me laugh, that both Yoda, and Obi-Wan had faced sith lightning, and yet neither one even thought to mention to luke, "Oh, and by the way, if he puts his hands up, like so, you might want to turn on your lightsaber")
I doubt that Luke at this stage would have been able to block Sidious's lightning anyway, even if he did know how. Taking into account that the Dark Lord wields it with such force that it physically ripped Yoda's lightsaber out of his hand in RotS.
True, and Mace was hardly able to stop it. Also, Kenobi just taught Luke the basics and then he died. And Yoda would likely have taught Luke a lot more, but Luke rushed away to face Vader in ESB before Yoda had taught him everything that he wanted to. And in ROTJ Yoda had so many things to say before he died that he couldn't teach Luke all the things Luke missed out on by rushing away in ESB.
Actually, the Return of the Jedi novelization details that Skywalker possessed the ability to hold off the Emperor's lightning for a little while, until Palpatine increased the output and overloaded his defense.
-You are about to face the evil who defeated Yoda twenty or so years ago. What does he say?
"Trained enough you are not, ****ed are we. Interested to see how this goes am I though, go and try, die you will but fun for the rest of us will it provide."
Because in ROTJ he was about 1 minute away from being dead. When he told Kenobi he was not powerful enough, there was a solution - Kenobi face Vader instead, and Yoda face Sidious. In ESB he told Luke he wasn't ready because there was a solution - Luke should stay on Dagobah and keep training so he is ready. In ROTJ Yoda had 2 options - to tell Luke he was ready, then die, or tell Luke he isn't ready, and then die. And what would have happened had he told Luke that he wasn't ready? Luke would have stayed on Endor I guess, and kicked some ass during battle there, Vader would eventually have confronted him there. Palpatine likely would have escaped from the Death Star before it exploded. The Rebellion might have even been destroyed before the Death Star was destroyed, since Palpatine wouldn't stop the STar Destroyers from firing to piss Luke off.