I'm doing my be best to imagine what you might be doing that would be similar to what you've done before, but could result in the issues described, on the additional assumption that iTunes is correctly installed on your computer. I, for instance, don't have any trouble adding media files to my library using the current version of iTunes, but there are places I could attempt to drop files on an open iTunes window that would be invalid.
The standard method with iTunes has been to add media to the library, then sync that to the device. At some point it seems iTunes has supported direct drag and drop from the file system to the device, indeed that was specifically added for adding apps to a device post iTunes 12.7, however it isn't clear that Apple ever meant for it to be used for audio files. The fact that it may have worked could have been a bug rather than an intended feature, and that it now doesn't could mean that bug has been fixed.
This comment from that thread appears to be salient:
I had the same issue, and I just STOPPED running itunes as admin, now it works. 😕
Which suggests that whether or not you can drag and drop direct from the file system to the device depends on which security account is being used for the operation. Either way you are using drag and drop from Explorer to device, rather than Explorer to iTunes, or iTunes to device, in a way that isn't explicitly catered for, and not one that normally comes to mind. The Microsoft Store version of iTunes will likely use different security accounts for operations as store apps are intended to be sandboxed for greater security. Should you wish to run the old school version of iTunes you can find direct installation links at the bottom of Troubleshooting issues with iTunes for Windows updates - Apple Community. Uninstall the store version first.
tt2