I am sorry about this issue you have experienced.
This quest is always troublesome. It usually pops out once you step on the stairs placed in front of Drake after you've defeated him.
If you haven't resolved the issue yet, please visit the Support Page
and file a Ticket to the Support Team in order to be assisted.
Please make sure to include as many details as you can.
If you don't feel confident writing in English, then feel free to use your own language if supported (Indonesian, Chinese, Filipino, Italian, Japanese, Portuguese, Spanish, Thai).
Thank you for your continuous support to the game,
If you have any other enquiries, just let me know and I’ll be happy to help.
See you in game!