Ghost can be made to run against MySQL 8 but you’ll need to adjust the password hashing and maybe some other things because MySQL 8 changed the auth defaults which are not currently supported by Ghost-CLI. https://github.com/TryGhost/Ghost-CLI/issues/833 may have some further information that could help.
If you’re unsure what you are changing, downgrading to an earlier MySQL version is probably the best route until MySQL 8 is fully supported.