This is most likely to happen if you enter the key manually. It is almost impossible not to make a typing error when the key is so long.
To avoid errors, try using copy/paste:
Place the pointer in front of the first character you wish to copy. Press and hold the left mouse button while dragging the mouse to the right until the desired text is highlighted. Right-click on the selected area and select "Copy". Right-click in the field where you want to insert the text and select "Paste".
See also: How Do I Activate My License Key?