Issue was in – robots.txt file is not writable.

Hello, I’m sorry to hear that you’re experiencing issues with your site’s crawling. The error message “robots.txt file is not writable” indicates that Rank Math is unable to modify your robots.txt file. To resolve this, you can add a filter to your site that will allow Rank Math to edit the robots.txt file. Here’s the code you need to add:

/**
 * Allow editing the robots.txt & htaccess data.
 *
 * @param bool Can edit the robots & htacess data.
 */

add_filter( 'rank_math/can_edit_file', '__return_true' );

You can also navigate to your wp-config.php file and search for the following code:

define ('DISALLOW_FILE_EDIT', true);

If the code exists, you should replace it with the below code. If it does not exist, add the below code to the wp-config.php file.

define ('DISALLOW_FILE_EDIT', false);

If you’re unable to access your wp-config.php file, I recommend reaching out to your web host for further assistance. Once these changes are made, you should be able to edit your robots.txt file through Rank Math.

If you have any questions or need further assistance, please feel free to ask.

Sources:
Common robots.txt Issues & How to Fix Them » Rank Math
I Can’t Edit My Website’s Robots.txt » Rank Math