When WordPress goes to update, it creates a lock in the database so that multiple users and processes do not attempt to edit the system in parallel. However, there are cases when this lock is created, but the update never occurs. If this happens and the lock is not released, you will be blocked from retrying until you manually clean up the lock.
The lock is created in the database in the wp_options table. You need to find the option with the option name core_updater.lock and delete the row. Then you can return to the site and run the update again.