Please open your php.ini file and set always_populate_raw_post_data to -1
ERROR : $HTTP_RAW_POST_DATA is deprecated from PHP 5.6 onwards and will be removed in PHP 7.0. Please open your php.ini file and set always_populate_raw_post_data to -1
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PHP Documentation
Your PHP Version is 5.6.23, but always_populate_raw_post_data = 0. $HTTP_RAW_POST_DATA is deprecated from PHP 5.6 onwards and will be removed in PHP 7.0. This will stop the installer from running. Please open your php.ini file and set always_populate_raw_post_data to -1. If you need more help please call your hosting provider.
How to fix the above error?
Error shows “always_populate_raw_post_data” should be set as -1. Log into your Linux Server via SSH as root.
If you want to change “always_populate_raw_post_data” serverwide then check the server PHP configuration file.
“php –ini” command will show the location of php.ini file. php.ini on the below server is /etc/php.ini, edit this file and change the entry “always_populate_raw_post_data = 0” to “always_populate_raw_post_data = -1”
Configuration File (php.ini) Path: /etc
Loaded Configuration File: /etc/php.ini

Turn always_populate_raw_post_data Off
Do the below steps if you want to set always_populate_raw_post_data to off only for your domain instead of disabling serverwide.
Create a custom php.ini file under document root of your website and change “always_populate_raw_post_data = -1”
[root@server public_html]# vi php.ini
Enter “always_populate_raw_post_data = -1” in php.ini. Save the file and exit.
[root@server public_html]# chown user:user php.ini
Create a phpinfo.php file under your website and make sure custom php.ini is loaded for your website. Check “always_populate_raw_post_data” in phpinfo file and make sure it is disabled. Now run magento upgrade and you wont see any errors.

Turn off always_populate_raw_post_data