Hi there,
I have not had to do this in a few years but it was caused by the Trader NPS being "lost" and falling down into the ground. He just was not there anymore so the post was closed. Spawning in a new trader used to work for me.
hit f1, type dm, hit esc, hit f6
spawn a temporary trader just inside the compound, hit f1, type dm off
you should be able to enter the trader if the trader is gone just do it again and spawn a static trader inside the compound.