The error "Fatal Error: Undefined Function is_network_admin()" in WordPress usually occurs when the function is_network_admin() is used in a context where it's not available. This function is typically used in a Multisite environment to determine if the current administration page is part of the network (Super Admin) or a single site.

Here are some steps to troubleshoot and fix this issue:

1. Check for Multisite Installation:

   • Ensure that you are running a Multisite installation of WordPress. If you're not using Multisite, the function is_network_admin() may not be available.

2. Check for Typos or Mistakes:

   • Double-check your code to ensure that is_network_admin() is spelled correctly and is used in a valid context.

3. Verify Function Availability:

   • In a non-Multisite installation, the is_network_admin() the function is not available. To avoid this error, you may need to use an alternative approach to achieve your desired functionality.

4. Conditional Check for Multisite:

   • If you're using is_network_admin() in a plugin or theme, make sure to check if the Multisite feature is enabled before using the function. You can do this using the is_multisite() function:
   php

4. if ( is_multisite() && is_network_admin() ) { // Your code here }

5. Update Plugins/Themes:

   • Ensure that all your plugins and themes are up to date. Outdated software can sometimes lead to compatibility problems.

6. Check for Plugin Conflicts:

   • Temporarily deactivate all plugins and try to reproduce the error. If it doesn't occur, reactivate each plugin one by one to identify the one causing the conflict.

7. Check for Theme Conflicts:

   • Temporarily switch to a default WordPress theme (like Twenty Twenty-One) to see if the issue persists. This will help determine if the problem is related to your current theme.

8. Review Error Logs:

   • Check your server's error logs for any specific messages related to the is_network_admin() error. This can provide insight into the underlying problem.

9. Contact Plugin/Theme Developer:

   • If the issue persists and is related to a specific plugin or theme, consider contacting the developer or support team for assistance.

10. Debug Mode:

    • Enable WordPress debug mode to see if there are any error messages related to the is_network_admin() function. Add the following lines to your wp-config.php file: