When attempting to install the theme I get the error message “The package could not be installed. The theme is missing the style.css stylesheet.”—What should I do?
You attempted to upload the entire product package. WordPress cannot determine which of the files contained in that package contains the actual installable theme. Please extract justlanded.zip from the archive, and upload that file instead.
You may find additional useful information in the Quick Start Guide, which is contained in the zip file you downloaded from themeforest.