If you somehow manage to cast Enter the Unknown when it's not your turn, the target creature explores when it resolves, but you won't be able to play a land that turn.
Enter the Unknown
If an ability instructs a creature to explore, its controller reveals the top card of their library. If it's a land card, they'll put it into their hand. Otherwise, they'll put a +1/+1 counter on that creature, then choose to either leave that card on top of their library or put it into their graveyard.
Once an ability that causes a creature to explore begins to resolve, no player may take any other actions until it's done. Notably, opponents can't try to remove the exploring creature after you reveal a nonland card but before it receives a counter.
Magic: The Gathering · ™ & © Wizards of the Coast · Illus. Howard Lyon
Card data via Scryfall. Rarix is fan-made; not endorsed by the rights holders.
