As of March 2nd, 2020, the Instagram platform will be deprecating its API. This will directly affect the Instagram feed section in all WordPress & Shopify themes. Read more on how it affects your theme here: https://fuelthemes.net/important-notice-instagram-feed-removal/

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I can't install theme - Stylesheet is missing

You need to first extract file you've downloaded from ThemeForest, theme is inside of this zip file along with documentation, license, psds and other files. You need to upload only theme folder to your WordPress.

This might be helpfull:
http://support.envato.com/index.php?/Knowledgebase/Article/View/269/0/my-wordpress-theme-isnt-working-what-should-i-do