Verify Epic->some filed contains value, when Task is set to completed. The task should not be able to be set to Completed, if the Epic->that field is not set.

[Neha_Saini]

is it possible to verify  Epic->some field contains value, when Task " is set to completed.

The task should not be able to be set to Completed, if the Epic->that field is not set.

[Niclas Sandstroem]

Yes but most likely you need a scripted condition. There might be a many plugins that you can do this with but I would do it with http://www.adaptavist.com/w/products-plugins/adaptavist-scriptrunner/

Good luck!