Fatal Error: Cannot redeclare wpseo_auto_load()
Fatal error: Cannot redeclare wpseo_auto_load() (previously declared in /srv/www/theme/wp-content/plugins/wordpress-seo/wp-seo-main.php:40) in /srv/www/theme/wp-content/plugins/wordpress-seo-premium/wp-seo-main.php on line 40
This error occurs when (free) Yoast SEO is already active at the moment you try to activate Yoast SEO Premium, since it's impossible to have both Yoast SEO and Yoast SEO Premium activated on the same website. So, if you want to use (and thus activate) Yoast SEO Premium, you will have to deactivate Yoast SEO first.
For more information on how to install Yoast SEO Premium, please visit: Installation Guide for Yoast SEO Premium.
Note: This also goes the other way around. If you have Yoast SEO Premium activated and you're trying to downgrade to WordPress SEO, you'll see the same error.