This article covers how to Fix WordPress Add Media Button Not Working.
The main reason behind this error is usually custom scripts not working correctly, plugins, or themes.
You can pinpoint the cause of your woes using your browser's Inspect -> Console.
The console tool highlights script errors that can help you to identify where the problem lies.
To fix this WordPress error:
1. That aside, start by deactivating all of your plugins.
2. Then check if the Add Media button is working.
3. If the button is working, then it is obvious one of your plugin is causing the script conflict.
4. Next, reactivate the plugins one by one, and check the Add Media button after each activation.
5. When you find the responsible plugin, contact the developer for help or find a replacement.
6. If the problem persists after deactivating all plugins, you need to ensure you've cleared your caches first and then test if your theme is the cause of the WordPress Add Media button not working.
7. Just activate a default WordPress theme such as Twenty Seventeen and then check if the problem is gone.
If the button works, the issue here is your theme.