When a Workflow is created, it is created in a draft state by default. In order to activate your Workflow, you first need to publish it. If you edit an active Workflow, the changes will not take effect until the Workflow changes are published.
Step by Step Instructions
Once your Workflow is complete, click "Publish Changes" in the top right of the editor to activate it.
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If you need to pause your Workflow, open it in the Workflow builder and toggle the "On" next to "Published Version" off.
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For more information on Workflow statuses, see: Workflow Status