The message seems to suggest you have the wrong ScreenOS platform download. Please verify the file is the one for the SSG550.
Also be sure your device has the new signing key as noted in these articles. The current downloads have a different key than a few years ago and your device might need this to be updated.
https://kb.juniper.net/InfoCenter/index?page=content&id=TSB16496
https://kb.juniper.net/InfoCenter/index?page=content&id=TSB16495