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This behaviour is by design. You cannot change or delete an item which is waiting for approval. You can edit and send an item for approval again if it is already Published or Rejected.
Custom workflow feature will allow you to make all kinds of changes and modifications in the workflow behavior. This feature is included in Professional and Enterprise editions of Sitefinity.
the Telerik team
Do you want to have your say when we set our development plans? Do you want to know when a feature you care about is added or when a bug fixed? Explore the Telerik Public Issue Tracking
system and vote to affect the priority of the items