If your virtual disk is at or close to the maximum size allowed by the file system, you might be unable to take snapshots due to overhead added by the snapshot process.  This failure occurs when the snapshot file at its maximum size would be unable to fit into a datastore. 

The failure depends on the size of the virtual disk. All virtual machines having disks with a maximum supported size by VMFS may experience this error. Overhead for the snapshot is roughly about 2GB for a disk size of 256GB. If snapshots are to be used, consider the overhead while deciding the size of the disks.  Follow the link below to view the maximum file sizes forthe different versions of VMFS.