You can assign a unique ID to every button and add them in an array.
Then randomize that array, that will be your unique sequence.
Now store that randomized array in other variable which you can use to compare and verify when the player is pressing buttons.
So when player is pressing the button add the assigned id of that button in a separate array
You can either compare it with the randomized array every time player clicks any button or compare it altogether when player clicks the last button