If you don't get any message, it means you type the serial number entirely wrong, or you typed it right, but with another product selected.
You need to insert your Serial Number. The Serial Number is indicated as "Serial Number" on your purchase receipt. It's not the Order Number or the SKU Number, it's the Serial Number, and it's only listed on your full purchase receipt.
Enter it exactly as it is, don't skip anything, including hyphens, and don't confuse the letter "O" with the number "0" and vice versa.