How to finish the Overseer's Mission?

I have 16/17 of the overseer's logs. The final one is Overseer's log - Fort Defiance. How do I get to that one?? Do i need to join BOS to get access? I'm full Enclave rn but I'm so lost. I thought I read I had to take my soldier's certificate to the DOL to get something to get in but i cant figure that out either pls help