I was getting the same issue here with a fresh version of 5.1.6. Looking through the admin/update.php it seems that the Database Upgrade prompt has been removed. I’ve hacked together a way of triggering this database upgrade.
https://gist.github.com/thisislawatts/89f17ab4075638a57657
* Replace the admin/update.php page
* Go to yoursite.com/wp-admin/edit.php?post_type=acf-field-group&page=acf-upgrade
* Revert admim/update.php to it’s original version.
This is triggered on each page load so don’t refresh it!
I would have liked to fix it up properly but I am little pushed for time this evening and just wanted to get my fields up and running.
Hope it helps.
