Hello,
The Pool shows a total space of 13.3TB, 2220GB of reserved space and allocated space of 11.5TB. This is leaving you 1.8TB of free available space. If as you say you are taking 1TB snaps there is not enough free space for you to take snaps when you take into consideration the snap and unseen/hidden snap.
If your allocated space is this large you are going to need to add more drives in order to add additional disk-groups to give you the needed space to take snaps.
Cheers,
Shawn