Is it harder to add multiplayer to an existing game than to make a new game with multiplayer?

I've been thinking of making a sandbox game, but I'm not sure if I should make the game then add multiplayer or make multiplayer while making the game

Developing multiplayer games is very difficult, and affects the design of the entire game. I would definitely recommend building from the ground up with multiplayer support, as it is probably too difficult to add multiplayer to an existing game.