The license key work is done if you see "Registered." But to ensure it stays working across reboots:
SoundSource gives you powerful audio control of your Mac, right from the menu bar. Rogue Amoeba SoundSource Manual — Purchasing a License - Rogue Amoeba soundsource license key work
Immediately contact Rogue Amoeba support via their official form. Include your order number. Their support team is responsive and will re-issue the key if there is a database error. The license key work is done if you see "Registered
a specific error message you're seeing while trying to enter your key? soundsource license key work
If you previously downloaded a cracked version of SoundSource that redirected activation servers to 127.0.0.1 , even a real license key will fail.