We have the following documentation about how to get Plastic working with Jira:
So, you can't get your task set as done after adding the attribute in Plastic?
Does other state transitions work for you?.
Please send us the following information in order to check it:
The repo you are working in. (If it is a local one, against cloud, on premise... )
The branch prefix and the proyect key
The plastic.debug.log.txt in the logs folder.