This third party extension is Protected, meaning it cannot be uninstalled. This is highly unusual, and most likely a mistake.
Protected extensions cannot be uninstalled. This is an attributed normally reserved for core Joomla extensions which must never be removed from your site, such as the user manager (com_users).
A third party extension which can never be uninstalled is, by definition, a mistake. Third party extensions must always be possible to install, update, and uninstall.
Onthos lets you unprotect the extension.
If you are a third party developer reading this, please DO NOT set your extension as Protected. It makes uninstallation impossible. If you think you know better than the person managing the site what belongs to the site and what not: you know nothing. Thank you for coming to my TED talk.