Hi...
This is your first topic, and if you requested for refund via Paypal then you do not need to report it here.
First of all 404 errors have absolutely nothing to do with "heaviness" of the theme. Most like you need to save permalinks in your site.
We provide free setup and installation of the theme and it usually takes 5-10 minutes and if 15k users are able to install it, I see no reason why you're unable to set it up on your site.
Although it is not clear if you were able to install the theme and use it. Your claims that you're unable to configure it and you never consulted with support staff ever and you've not mentioned a single question in this topic as well.
Lastly, please check your server requirements, 99.99% of users facing such issues are because of "poor" server configuration which is clearly and very clearly mentioned in the "FAQ" section which you should check before purchasing any product, refer the FAQ section here:
https://themeforest.net/item/wplms-learning-management-system/6780226/support
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For refund you can also create a ticket on "support.envato.com"