Best for: "Unidentified Developer" errors.

Enter your Mac password and confirm that you want to open the installer. Run the installer again; it should now proceed normally. 2. Download Incomplete or Package Corrupted

The most common "download error" is actually a macOS security feature. Because Epubor is not distributed through the App Store, macOS may block the .pkg file with a message saying it "cannot be verified" or is from an "unidentified developer."