I can't install theme - Stylesheet is missing

If you have download the full package you need to first extract the file  you've downloaded from ThemeForest, the theme is inside of it along  with documentation, license, and other files. You need to upload only  theme folder to your WordPress. This might be helpfull:

https://help.market.envato.com/hc/en-us/articles/202821510-Theme-is-missing-the-style-css-stylesheet-error