WordPress 4.9.3 was released earlier this week and developers tried to reduce the number of API calls that occur when an auto-update job is run.
#43103-core aimed to reduce the number of API calls which get made when the autoupdate cron task is run.
This resulted in a bug that causes a fatal PHP error when WordPress tries to update itself.
Unfortunately due to human error, the final commit didn’t have the intended effect, and instead triggers a fatal error as not all of the dependancies of find_core_auto_update() are met. For whatever reason, the fatal error wasn’t discovered before 4.9.3’s release – it was a few hours after release when discovered.
Updating to the new 4.9.4 is release will fix the bug and your WordPress core will be able to auto-update as it should.
WordPress 4.9.4 Maintenance Release Details – View