You are in a painful position, you have two options,
1: add an extent to the datastore and grow your size above 2TB, this will give you the space to commit the snapshop
2: perfrom a V2V conversion on the guest machie to another more empty datastore.
Snap shots are not a back up methodology, they are a short term panacea to allow dangerous tasks to be undertaken safely ie a service pack upgrade or application upgrade whcih allows a point in time roll back to before the change was made or for backup products to make a full backup of a running VMDK without issues caused due to locking.
the only valid long term use of Snapshots is for Linked Clones, but this is a special use case that requires special VMware software to undertake.
