If your Lightroom 5 key is an "Upgrade" version, you will likely need the serial key from your previous version (e.g., Lightroom 4) to complete the activation.
Are you trying to for a version you already own, or lightroom 5.7.1 serial key
Just wondering how I input my serial number into LR 5 | Community If your Lightroom 5 key is an "Upgrade"
It includes the classic "Develop" module features like the Advanced Healing Brush , Upright tool for perspective correction, and Radial Gradients . Upright tool for perspective correction