proc_open() has been disabled for security reasons
Your host has likely added proc_open to your php.ini file's disable_functions setting.

If you have access to it, you can remove it from that setting.  If not, you'll have to ask your host to do it.

You can see that Craft requires the PHP proc_* methods to be able to perform some operations here: https://docs.craftcms.com/v3/requirements.html#optional-php-methods-and-configurations