This is typically caused by a theme author having done an incorrect search & replace in the TGMPA file.

The simplest way to get round this, is to replace the file.

1. Download the current version of the library from http://tgmpluginactivation.com/download/ (for your purposes, just click on the “Zipball” link below the form)
2. Unzip the file and
3. Copy/upload (via FTP) just the class-tgm-plugin-activation.php file to the /public_html/wp-content/themes/xx/includes/plugins/class-tgm-plugin-activation.php directory of your website, replacing the existing class-tgm-plugin-activation.php file.

Once you’ve done that, all should be fine again.

All the same, please contact the Theme author and ask them to update the TGMPA file they include with their theme. They should download a fresh copy of TGMPA using the Custom TGMPA Generator, which will serve them with a customized download of TGMPA which will automagically have done the search & replace the correct way.

Hope this helps.