I get an error of type E_ERROR from class-tgm-plugin-activation.php

You are getting this error because both your theme and the plugin are loading the TGM plugin class. It looks like the theme you are using is not checking if this class has already been loaded.

Add the following to your child theme's functions.php file so you only load the theme's version:

add_filter( 'gpur_load_tgm_class', '__return_false' );