Wensam, how did you get the version of LR5 you're trying to enter the serial number into? If it was from a download you may have downloaded the CC version by mistake (easily done as it's harder to find the standalone download).
If this is the case you'll need to un-install the version you have at the moment and download one of these two:
For Mac:Adobe - Lightroom : For Macintosh : Adobe Photoshop Lightroom 5.6
For Windows: Adobe - Lightroom : For Windows : Adobe Photoshop Lightroom 5.6