I competed the undersea facility 6th birthday, but it's not giving me the last piece. Please help me to resolve it. Thank you for your help.
Don't know what you mean that it won't give you the last piece? If you can't find a tile, try getting the Last Tile Finder from the Shop>Extras.
If you're referring to the key. You have to turn on all the fans in the room with the key.
If you're referring to the Treasure, you have to point all the lasers at the stone & then the Treasure piece is revealed.