File Drag and Drop only works on records and transactions that are in the old NetSuite UI. Ensure the preference to show the project in the new NetSuite UI is disabled. Go to Setup > Company > General Preferences and make sure the Show Project In New UI box is unchecked.
File Drag and Drop enables you to upload single or multiple files from your desktop to the file cabinet or to a record page. It can also be used to attach multiple files directly to editable sublists of supported records and custom records. To learn how, see Using File Drag and Drop.
Your administrator must install and configure File Drag and Drop before you can use it.