(Please try to use more descriptive thread titles.)
Dragging a file should work, but if you're on macOS, note that .epub files from some tools might actually be macOS packages (i.e., specially marked folders). Zotero can't import those. You should be able to convert those to actual single files, though.
If that's not the issue, we'd want to see a Debug ID for an attempt that doesn't work.
Upgrade Storage
Dragging a file should work, but if you're on macOS, note that .epub files from some tools might actually be macOS packages (i.e., specially marked folders). Zotero can't import those. You should be able to convert those to actual single files, though.
If that's not the issue, we'd want to see a Debug ID for an attempt that doesn't work.