Some software requires exclusive permission while opening the file. In most cases, the file is saved, but not closed in WinISO.
Since the file is still opened in WinISO, the other software may fail to open the ISO because it cannot get an exclusive permission.
In such situation, you can choose 'Exist' in 'File' menu or simply close WinISO and try again.
This usually happens when you opening other disc mounting software simultaneously, e.g., Daemon Tools on Windows, ISO mount on Mac OS. And it only simply says 'cannot open', which may lead us to consider if the ISO image is damaged.