Did you release the slaves at a Commonwealth station? You need the right number of coffins to complete the mission.

Yeah, you right, I lost coffins...
I attach file. Two ways: dock with Huary fortress on Avenger-class ship, or buy Corvet-class ship on Straton and dock with Huary. On Corvet-class I cannot complete mission. I need get coffins to complete in this way?

P.S. I wrote many words there, after I attach file, and all my text cleared!

I'll have a look. The git code looks like it should fail the mission if the player doesn't have the slaves. It might be a 'register the playership for events' bug which happens when the player changes ships during missions.

EDIT: Some testing:
Accept the mission, get the slaves, jettison the slaves, dock with the Huari fortress = mission failure.

Accept the mission, get the slaves, jettison the slaves, change playerships, dock with the fortress = inProgress text.
Change back to the first playership and dock without the slaves = mission failure.

Accept the mission, get the slaves, jettison the slaves, change playerships, reload the slaves into the hold, dock with the fortress.
This shows descInProgress.someSlavesLeft with zero as %slavesLeft%. The slaves are removed from the new playership and vanish, the empty Sung transport wreck is set as the player target but the mission doesn't succeed.

Might be the (= aObjDocked gPlayerShip) code in OnObjDocked not working appropriately for the new playership. The code seems to be working for the InProgress text and OnInProgressUndock.

Whenever you dock at any place where you can buy ships all the slaves will automatically be freed. You might get a little reward for this. But you cannot finish the mission anymore.

 You can also get a new ship from Corporate Shipyards and Black Market Shipyards — neither of which free slaves — so you’d need to see if switching ships that way still breaks the mission if you still have the slaves.