Http error 500

Any help.
I can´t install the script because i get error HTTP ERROR 500.
Check the logs and get these: PHP Fatal error: Call to undefined function Grav\Common\Config\json_encode() in /var/www/…/data/www/…/system/src/Grav/Common/Config/Compil edBlueprints.php on line 3

any reply!

I think you have a very old version of PHP. You need PHP 5.5+ for Grav.

No I can change the to any I whant! I have tried and nothing.