I can’t install the MainWP Child plugin. When installed on a website, the plugin disappears from the list of plugins and cannot be activated or deactivated.
The only thing I have been able to do is to enter via FTP and delete it manually, but this prevents us from installing the plugin to be able to associate more sites to our account.
As @josklever pointed out, the plugin was likely hidden by a previous user using the White Label extension.
Please follow the steps from the KB that Jos provided and let us know if that helps.