Why can’t I add or edit time boxes on the organization roadmap?
Because you are looking at it through one product. Organization time boxes are organization-wide, so they can only be changed from the view that shows the whole organization: All products.
An org roadmap’s columns are shared by every product in the organization. Renaming “Q3 2026” or deleting a column from inside one product’s slice would silently change what every other product sees, from a screen that gives no hint that anyone else is affected. Restricting the edit to the all-products view makes the scope of the change obvious.