Justices need to die or retire in order for there to be a vacancy for a President to appoint a new Justice to. There was a vacancy at the start of Trump's term due to a death during Obama's that the Republicans refused to confirm an appointment for, and then there was a retirement (Kennedy) and death (RBG) during his term as well.
Something that'd be significantly easier to do if it weren't for the US's continued immoral blockade and sanctions? Cuba does not have the resources to maintain the plant purely on their own, and they can't hit a magic button that will suddenly let them find an abundance of spare tools and equipment to repair their infrastructure in a disaster scenario without wider access to international trade.