Do you know what version of PHP you are using? I suspect that perhaps the PHP version is fairly old. You can find this information in cPanel or your hosting provider would know it. Or you can also install the “Display PHP Version” plugin and it will show your version in your WordPress dashboard in the “At a Glance” section. Then just delete the plugin once you know your version.
WordPress recommends a PHP version of 5.6 or greater. The fastest solution may be to upgrade PHP if you’re on an older version.
Were you able to confirm the version of PHP running on your server? It’s possible that this issue is something that can be fixed in the plugin itself, but it would almost certainly take longer to resolve than upgrading PHP itself.