It is necessary to add Modify Building permission for players to be able to build inside a building.

On a settlement you need to enable the Pick Up permission for a player to be able to use a hitching post. For non-citizens create a new roll. Settlement, Manage Roles, Add Role (upper right), enter a Role Name (I typically use the player's name) in the first field and the player's name in the Player (third) field. Check the permission box for Pick Up and Save.