Hi currently MobileFrontend doesen't support extension.json but is in the process on being converted to it. But I think you may have set
wfLoadExtension( 'MobileFrontend');
Please follow the installation instructions at
https://www.mediawiki.org/wiki/Extension:MobileFrontend#Installation
Please follow installation instruction for EmbedVideo at
https://www.mediawiki.org/wiki/Extension:EmbedVideo#Installation