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It's not an error, but just a warning. Turn off debug mode by editing /config/defines.inc.php and changing this section:

/* Debug only */
if (!defined('_PS_MODE_DEV_')) {
	define('_PS_MODE_DEV_', true);
}
/* Compatibility warning */
define('_PS_DISPLAY_COMPATIBILITY_WARNING_', true);

to this:

/* Debug only */
if (!defined('_PS_MODE_DEV_')) {
	define('_PS_MODE_DEV_', false);
}
/* Compatibility warning */
define('_PS_DISPLAY_COMPATIBILITY_WARNING_', false);

This way warning won't be showing up, but errors will break your store and show blank page. Above settings should be true only in development. The moment you set your shop on production site these settings should be changed to false.