I have created some letters in AEM Forms using Correspondence management feature in AEM Forms 6.3
But Submit button is not getting enabled to submit it to a workflow even though I have selected a Post process workflow.
Have you configured the Manage Letter Instances on Publish setting on the Publish instance?
See Manage content in preview and submit the letter (6)  and
CM Configuration Properties 
 Create Correspondence
 Correspondence Management Configuration Properties