This usually means the PKG was not merged correctly (if split) or your USB drive has errors. Re-download the PKG, reformat the USB to exFAT, and ensure your jailbreak is active. Also, verify that the PKG is for your firmware version (e.g., a 9.00 PKG may not work on 5.05).
The PS4 version utilizes robust emulation to ensure the games look and play exactly as they did in the arcade (MVS/AES). Metal Slug Ps4 Pkg
You can find it on the US PlayStation Store for $19.99 . 2. Metal Slug XX This usually means the PKG was not merged