If you can’t activate Realteo plugin and WordPress returns following error then you certainly have outdated PHP version on your server. PHP should be at least in 5.6 version. If you are not able to update it on your own contact your hosting provider and ask them for an update.
You can check your current server PHP version by installing this plugin: https://wordpress.org/plugins/display-php-version/
After activating that plugin you’ll be able to see current version of PHP in your dashboard: