I am unable to edit predefined block like social media. I have also tried about us block but it displays error as:
"There has been a critical error processing your request
Please check AbanteCart and webserver error logs for more details. You can check error log in the control panel if it is functional. Otherwise, refer to error log located on your web server"
Perhaps your file permissions are not set correctly as I see you are new, so assume your install is new too.
Read the file permissions on this doc then verify your files are set correctly
http://www.abantecart.com/ecommerce-documentation/user-manual/installation (http://www.abantecart.com/ecommerce-documentation/user-manual/installation)
If that isn't the issue - please post the error log so we can see the 'More details"
chmod your folders to 666 file permission