Hi Kumar,
The "more intervals" button will only be seen when new depreciation engine is actived. You can check it in table TBE11 for field EA-FIN to see if X is marked for it.
Besides, if the asset that you want add interval is a leased asset In this, in most of the cases leased assets will use old calculation logci, which means no interval can be added.
Please read notes 1393973 and 1155771.