The error message might big, but the solution is so tiny.
This error is an outcome of degraded file permission level. Due to the 640 level permission to wp-config.php, the WordPress themes, plugins fails to change/save. Just change the file permission of wp-config file to 644. That's it. Done..! 
How to do that,
- You can view file permission level via cPanel, FTP, FileZilla or CyberDuck
- Change the file permission to 644 for wp-config.php
- Done..!
UPDATE:
With reference to comment of Soundarya, we need take consider to check file permission of wp-config.php too (along with .htaccess). If you experience error as illustrated below, then probably it denote to check file permission of wp-config.php
AH01215: PHP Fatal error: require_once(): Failed opening required '/home/username/public_html/wp-config.php' (include_path='.:/opt/cpanel/ea-php73/root/usr/share/pear') in /home/username/public_html/wp-load.php on line 37: /usr/local/cpanel/cgi-sys/ea-php73
The front-end error somewhat like below.
In this case you need to change file permission of wp-config.php level to 644.
