Seems that you are uploading wrong .zip file. You should unzip package you downloaded from themeforest/elements and you will find another .zip in it, which should be called like your THEM-NAME.zip. Please try to install that .zip
please make sure you have purchased shopify theme. also you can check theme documentation where author should explain how to use/install the theme.
Otherwise the best way to get the support is Contact your purchased theme Author. How to Contact Author and let them know. theme Author will be very happy to assist you.