When attempting to do a quiesced backup, it fails with "Error Message: Unable to continue Differential backup; the quiesced snapshot was not found."
The Virtual Machine that you are trying to backup is a linked clone.
The backup of linked clones is not supported at his time. In some instances the backup may work, but it may fail in a number of scenarios. One scenario being: Full quiesced backup is successful, subsequent quiesced differential or incremental backup fails with similar error message.