Hi,
I agree with JCMorrissey, its problem with virtual disk descriptor file, from datastore browser we should not be seeing flat disk, in your case we see it, that mean, virtual disk descriptor file is corrupt.
Connect to ESXi host using the SSH, on which this VM is register from, then run this command " touch test" inside the /vmfs/volume/nfs_datastore it will tell whether we have permission to write on this NFS volume or not.
Also we need to see where you are placing the snapshot, since its complaining about size of disk , which you are trying to creat. Since in NFS, it should not matter, unless you are trying to create virtual disk of size larger than NFS volume mounted on ESXI host.
Regards
Mohammed