cannot mount '/tank': directory is not empty
So you should zpool export tank, and make sure it is exported(!) then remove entries that are in /Volumes/tank (or /tank depending on version), as they are in the way of ZFS. Usually this is empty dirs where other items are mounted, but you may have copied something there
at some point when the pool wasn't mounted. When the directory is empty, everything should go back to automatic. ZFS requires the directory to be empty so that it doesnt accidentally cover files you want/need.
or just zpool import tank - again.