Next you can head west of Violet City and run into a cave entrance. Inside will be a puzzle that you have to put together. Once you discover it was Kabuto, the floor will drop out and down you will fall into the cave below. (If nothing happens, you put the puzzle together wrong) Inside are a bunch of Pokémon called Unown.
This is the answer to the puzzle. Make sure it’s the exact same with proper swapping of identical pieces. If it still wants to you resume the puzzle, then something is off. Try again. Don’t be discouraged. Sometimes you need to rotate the eyes or something.
There’s no known glitch or bug; you just have it wrong. If you’re 100% positive you’ve tried every possible combination, save and turn off the game, and come back to the puzzle tomorrow.
Source: experience, serebii