The 4.7.x series of IPB have all REQUIRED PHP8. So if you're running the latest version, it's absolutely compatible with it. Either you're missing a PHP module as noted by Marc above... or you have a 3rd party resource (application or plugin) that is not compatible with PHP8.
If Marc's check above comes back clean, I would suggest disabling all 3rd party resources and testing. If you can't login to your ACP, use recovery mode to disable everything for you.