If you're having trouble uploading media to Twitter, it could be for a number of reasons. Here are some potential causes:
1. Your file might be too large. Check the file size and make sure it's less than 5MB.
2. You might be trying to upload an unsupported file type. Right now, Twitter supports JPEG, GIF, and PNG files.
3. There could be an issue with your internet connection. Try restarting your router or modem and then try uploading again.
4. Make sure you're using the latest version of your browser. If you're on an older browser, that could be causing problems with media uploads.
5. Finally, try clearing your browser's cache and cookies and then reloading the page before trying to upload again