If you have disabled System Integrity Protection (SIP) or run a heavily modified macOS, the license validation checksum may fail.
When you purchase SoundSource from the Rogue Amoeba website, you receive a license file or a license code (usually via email). Here is the technical breakdown of how the key interacts with the software: soundsource license key work
When users ask "does the soundsource license key work?" — 99% of the time, the answer is yes, provided the key is valid and entered correctly. If you have disabled System Integrity Protection (SIP)