This error means that your theme doesn’t support thumbnails, in order to add this option please find the file ‘functions.php’ in your theme and add the following strings to this file:
add_action( 'after_setup_theme', 'theme_setup' );
function theme_setup() {
add_theme_support( 'post-thumbnails' );
}
This article related to Portfolio by BestWebSoft plugin
Additional resources: Portfolio - User Guide
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