Er, that really doesn't tell us enough to help you.
What's the error, and what does your post-function try to do? What are the users entering in the field and how are you translating it into the format the original estimate expects?
I created a Text Field (< 255 characters) and in a workflow transition I am using Copy Value From Field to Field post-function for copy the text field value to Original Estimate Field, but jira returns the error message "Unable to copy value from 'Especificação Funcional' to 'Original Estimate'
It seems that you have tried to perform an illegal workflow operation.
If you think this message is wrong, please contact your JIRA administrators."
I found similar reports, but with diferents fields:
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