Perhaps use the “Upload” button in Files, instead of drag-and-drop… End of the day, the package MUST be in the root folder, otherwise it will not be found after a reboot to install – at boot ONLY the root directory / is searched for *.npk files.
(If this is a test system… you can use 7.18beta, which lets you do this from Sytem > Package without uploading anything. In 7.18, you can do a “Check for Updates”, and in System > Package list you should be able see and enable user-manager, then reboot again for it to be installed)