Fallout 4: bug in minutemen mission

The relevant quest should be Returning the Favor. First of all, try to speak with Blake Abernathy at the Abernathy farm just south-west of Sanctuary. He is the one giving details of the quest, namely to take revenge on the raiders for killing his daughter, but also to return her stolen locket. If you haven't returned the locket, this could be why the quest isn't finished.

It is found at the bottom of the Satellite station inside a toolbox, before the storage room filled with radroaches - including a radroach boss.

However, to unlock the final mission with the Minutemen, you only need to hold a certain amount of settlements for the Minutemen faction. You also need to be hostile with The Institute. (I believe you can also optionally have the Minutemen go to war against the Brotherhood if you are hostile to them.) All of this assuming you have finished all Minutemen quests such as reclaiming the castle and later defending it, and that you have found the Institute through the main story line quests.

Go talk to the settlers at Abernathy Farm (go here for location http://fallout.wikia.com/wiki/Abernathy_farm) and talk to the people until they tell you to go to Olivia, then go back to the station and head to the very back where there is a red toolbox in front of a pair of double doors that leads to the last room. There is a locket in there that you need to return to Mr. Abernathy. Do all that and you should complete the quest.