Gonna sound like an idiot for asking but, where is that exactly?
In between inventory and radio. You need to be in one of the following cells:
-Public cell
-Cell owned by you, in build/elite mode, not sure if teams are affected by this.
-Cell owned by mutual friend, in build/elite mode, not sure if teams are affected by this.
-Cell owned by team, in build mode, and you are given team building permission.
If you're not allowed to build in the cell, the button will not show up.