Did you download the creative cloud version, and now you are trying to register it as a standalone version?
What operating system are you using?
What version of Lightroom are you trying to install?
Have you had previous versions of Lightroom?
Is this version that you're trying to install an upgrade version?
Do you have your Lightroom registered with Adobe so that you can check your serial number on your account?
Again, the more details you can give on what you have done and what has failed, any error messages received, etc. will speed up this process of trying to provide assistance to you.
Also, is the serial number 24 digits with (for LR 5) the first four digits being 1160? And where and how did you acquire the number?
Welcome to the Adobe Forums!
If you downloaded the product via the Creative Cloud, you won't be able to activate the product with a serial number.
To install Adobe Photoshop Lightroom 5 without the Creative Cloud, please follow the link below:
If still you have done these steps already and the serial number can't activate the product, please let me know what is the exact error message.