A total and complete sign off from White House Doctors

Yes.

"It" refers COVID-19. By putting "immune" in parenthesis the phrase "that means I can't get it," is being interpreted as meaning, "I am immune to COVID-19."

Does "sign off" mean "offficial order of discharge"?

You have to remember orister, that Donald Trump speaks his own native English at the level of an 8 year old.

In everyday language, "signing off" is informal. It means basically, gave approval for, moved it along, etc.

by the way, he isn't in fact "immune" from anything. He can get it again. Further indication you're dealing with someone who has little knowledge in general, let alone of English.

Problem with you uneducated people is that you literally do not understand any science except what is spoon fed to you by the media. Yes Trump is immune, for a few weeks, because he has developed antibodies to fight the virus. This is the very same protection that a vaccine provides.

The problem is only that the antibodies for SARS-CoV-2 are typically short lasting (only a lasting a few weeks to a few months) for those with a fast recovery, not that they do not provide protection against reinfection.

It really should be noted that "immunity" isn't the Hollywood standard where a magical force-field prevents the virus from entering your body. I find it hilarious how people don't recognize that a vaccine does not prevent a virus from entering your body, it merely gives you the tools to fight it off. The same goes for SARS-CoV-2... the whole idea is that you remain asymptomatic while infected regardless of how you acquire the immunity.