Order 6 is one of the earlier missions you will find in the waystation near Capital Knot City.
Your task is simply to build a postbox at the designated location and you’re done.
Here’s a walkthrough of Order 6 in Death Stranding.
Head towards the postbox construction site
Once you have a PCC, go towards the postbox construction site as indicated on your map.
There may be Beached Things (BTs) nearby but the postbox area won’t have them.
Build the postbox
Enter the circular area which is the postbox construction site.
Use your menu to choose PCC. Hold L2 and you can switch to the relevant structure. Choose the postbox and press R2 to build it.
It should take a couple of seconds for the completion.
And the mission Order 6 ends here.