If you want to build structures in Death Stranding 2, then you need to make sure that you’re in a zone that’s connected to the UCA’s Chiral Network. That means completing a few missions/orders until the NPC leader of the settlement says that they’d like to be connected via the Q-Pid.

Zones that are part of the Chiral Network have a blue border seen when you scan the area. Those outside the network have a red border.

Furthermore, each structure takes up a certain amount of the zone’s bandwidth. As such, you have to level up your connection/reputation with settlements to increase your total available bandwidth.

Note: The bandwidth mechanic is somewhat circumvented due to the number of online players. That’s why it’s possible to see numerous structures within a given zone–other players are plopping down their own devices, and you’re able to use them, too.