The "Cannot Modify Header Information" error in WordPress is typically caused by output being sent to the browser before the headers are set. This can happen due to various reasons. Here's how you can troubleshoot and fix this issue:

1. Check for Whitespace or Output Before <?php or After ?>:

   • Open your theme's functions.php file and make sure there is no whitespace or any other content before the opening <?php tag or after the closing ?> tag.

2. Check for Blank Lines in Files:

   • Sometimes, blank lines or spaces after the closing PHP tag ?> can cause this issue. Make sure there are no spaces or lines after the closing tag in your PHP files.

3. Avoid Using echo Before header():

   • If you're using the header() function to set HTTP headers, make sure there are no echo or print statements before it. Headers must be sent before any content is output to the browser.

4. Check for BOM (Byte Order Mark):

   • If you're using a text editor, make sure it's not adding a BOM to your PHP files. This can sometimes cause header errors. Use a text editor that allows you to save files without a BOM.

5. Disable Plugins and Switch Themes:

   • Temporarily switch to a default WordPress theme like Twenty Twenty-One and deactivate all plugins. Then, reactivate your theme and plugins one by one to identify if any of them are causing the issue.

6. Check for Extra Spaces in wp-config.php:

   • Open your wp-config.php file and make sure there are no extra spaces or lines before the opening <?php tag.

7. Use Output Buffering:

   • You can try using output buffering to capture the output before sending headers. Add ob_start(); at the beginning of your PHP file and ob_end_flush(); at the end.

8. Check for Redirect Loops:

   • Sometimes, a redirect loop can cause this error. Make sure your .htaccess file and any plugins or custom code are not causing a redirect loop.

9. Review Plugin or Theme Code:

   • If the error is related to a specific plugin or theme, review their code for any instances where headers might be getting sent prematurely.

10. Check for PHP Errors in Logs:

    • Access your server's error logs to get more information about what's causing the issue. Look for any specific PHP error messages that might provide clues.

11. Contact Hosting Support:

    • If none of the above steps work, get in touch with your hosting provider's support team. They may be able to provide specific advice based on their server configuration.

12. Seek Professional Help:

    • If you're not comfortable troubleshooting PHP issues, consider hiring a professional WordPress developer or consultant to assist you.

Remember to back up your site before making any major changes, especially when troubleshooting issues like this. This ensures you have a safe point to revert to if anything goes wrong during the process.