Check for Whitespace or Output in wp-config.php
1. Open your wp-config.php file.
2. Look for any whitespace or blank lines before <?php or after the closing ?>.
3. Remove any extra spaces or new lines.

Debugging Mode

Add this to wp-config.php:
define('WP_DEBUG', true);
define('WP_DEBUG_LOG', true);
define('WP_DEBUG_DISPLAY', false);
Check the debug log at /wp-content/debug.log for PHP warnings or errors.
OR
adding ob_start(); at the start of wp-config.php

.htaccess Issues
Rename .htaccess to .htaccess_backup and regenerate it by going to Settings > Permalinks and saving.

Remove BOM
Ensure that none of your PHP files contain a BOM at the beginning. The BOM is an invisible character that can interfere with HTTP headers and trigger the "unexpected output" error.

• Changing file formats (UTF-8 BOM to UTF-8)

Deactivate Plugins
Disable all plugins to check if the issue is resolved. Reactivate them one by one to find the culprit.

Update Plugins
Ensure all plugins are updated to the latest version.

Theme Issues
Switch to a default theme (like Twenty Twenty-One). If the error disappears, the issue is with your theme.