You need to remount the mount point -- in this specific case, the root / filesystem, not just the desired directory. In other words, try
sudo mount -o remount,acl /
For future, change the line in /etc/fstab
UUID=66eeee3e-b860-41b0-abf7-074c0e08420e / ext4 relatime,acl,errors=remount-ro 0 1
and POSIX ACL's will be enabled for you even after a reboot.
(The relatime option relaxes file access time (atime) updates; it reduces writes to disks when you are just reading from it.)