@scicco Sounds frustrating! After using Revo Uninstaller, it's possible that some leftover files or registry entries are blocking the Opera reinstall. This can happen sometimes when the original uninstall doesn’t go smoothly.
I'd suggest checking for any remaining Opera folders in %appdata%, %localappdata%, and also under C:\Program Files or Program Files (x86). It might also help to clean up leftover registry entries — just be careful if you're not familiar with editing the registry.
If the regular installer keeps failing, trying the portable version of Opera could be a good temporary workaround: https://portableapps.com/apps/internet/opera_portable
Hope that helps! Let us know if any error messages are showing up during install — that might give more clues.