Basically Nirvana is trying to load the same plugin that GM Neo has already loaded. We’ve added a check on GM Neo side, but this issue will still happen since the error message you quoted above suggests that the Nirvana is loaded after GM Neo is loaded.
Try asking the Nirvana developers to add a check before loading TGMPA.
if ( ! class_exists( 'TGM_Plugin_Activation' ) ) require_once( 'includes/tgm.php' );
This free Support Forum relies on contributions from our support staffs and fellow Graphene themes users. While we endeavour to provide adequate and timely support, due to its voluntary nature responses are not guaranteed and may be delayed.
If you require urgent assistance, you may like to consider our Priority Support option.