How’s your Allow Building setting in Island Settings, Teams Setting & Class Designers ?
Are you inside a large mutator ? check it’s Allow Building setting
Another one is hitting the builds limit, have you used too many player builds in making your map ?
The ground, are you on a custom asset or model ?
When terrain doen’t allow building on it ?
(Can’t remember if its correct or not, but placing a grid plane underneath the terrain makes it work ???)