If your key has been publicly posted on a forum or keygen site, Serif’s (now inactive) blacklist may have flagged it. Again, with servers down, this is irrelevant—but the local installer may still reject it.
If you find the legacy software too outdated for modern operating systems, Serif often provides information on transitioning to the newer Affinity apps. serif legacy product keys
After installation, legacy products often prompt for a "Registration Code" (distinct from the initial Product Key). Since the online and phone registration servers are now offline, Serif has provided a Universal Registration Key to bypass this prompt: The Universal Code Affinity | Forum If your key has been publicly posted on