error assigning organisation structure

[priyank dev]

Hi experts,
                I have cerated an organisation structure (staff assignments)
and assigned some users to it.This organisation structure had
been assigned to a  workflow.So when ever a notice is created for employees
it should be sent to all the users assigned to that orgn structure.I have
created a mail and assigned(orgn structure)  to it.When ever i execute my
workflow its showing error-Work item 000000399133: Object 000000399133
method SENDTASKDESCRIPTION cannot be executed.
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