Opera blacklisted the version of Tampermonkey that is currently offered on the Chrome Web Store as it is being installed by Windows malware. This prevents the extension from working in the Opera browser. It is not uncommon for malware, especially adware bundles, to manually install malicious extensions in this way. If you are convinced the version is harmless and require that specific one, you can remove the blacklist by editing Opera’s Preferences file. You can do this by following the instructors offered by TechDows.
Source: https://www.bleepingcomputer.com/news/software/opera-blacklists-tampermonkey-extension-being-installed-by-malware/