There’s a pretty nasty bug in Civilization 6 on the Nintendo Switch that prevents winning. According to reports from numerous players, the game has a tendency to crash just before the human player gets to declare victory. Like a sore loser at checkers, the game just quits when you’re about to win. Thankfully, there’s a workaround while 2K Games figures out a fix.
“A discovered known workaround is to declare war on the Civilization that is causing the crash right before you end your turn.”
There’s your solution for now. Crash out, then reload and declare war on the civ that caused the crash. You’ll feel the sting of the AI’s tantrum, but you’ll get to rub it in Civilization 6’s face eventually.