Cisco tightly controls access to its software repositories. You cannot simply find a public mirror. Ensure you have:
Do NOT download ASA 9.12.4 if you cannot register Smart Licensing. The device will effectively become a basic NAT/firewall only. asa 9.12.4 download
: If you are coming from a very old version (e.g., 9.1 or 9.2), you might need to stop at an intermediate "step" version before reaching 9.12.4. Cisco tightly controls access to its software repositories
: Always export your running configuration before initiating an image upload. 9.1 or 9.2)