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How to install theme on WordPress.com service? I can't install theme on WordPress.com

You can't install Themeforest and any other marketplaces themes and plugins (any themes/plugins from ZIP files) on WordPress.com service, because this is not a real hosting.

Read more here: https://en.support.wordpress.com/themes/adding-new-themes/

WordPress.com and WordPress.org is two different things: https://premium.wpmudev.org/blog/wordpress-org-vs-wordpress-com-2015/

You need to purchase any good regular Linux Based shared hosting with CPanel and point your domain to it (change NS servers on WordPress.com side for your domain) or register new domain if you does not have it. Then you can install WordPress from hosting CPanel with few clicks (or manually using installation package from WordPress.org) and install theme by documentation.

Check our hosting recommendations for WordPress installations here: http://creanncy.com/hosting

If you purchases our recommended hosting by our special promo link we will install WordPress and your theme with demo data on your new hosting for free.

This is your (buyer) responsibility to have hosting and domain if you are working with WordPress and unfortunately this is not a valid reason to get a refund by Themeforest rules if you does not know that you must have real hosting for installing custom themes, because this issue does not have any relation to product itself that you purchased.

If you have any questions about Themeforest refund policy you can contact Envato Support at http://support.envato.com/