Audio file upload for Voice Cloning is available on the Starter plan and above.
On the Free plan, only the script-reading (recording) method of Voice Cloning is supported; the audio file upload method is not provided. So on the Free plan, the file selection window not opening when you click the upload button is normal behavior, not an error.
To use Voice Cloning by uploading an audio file, please subscribe to the Starter plan or higher. ('Voice Cloning via Audio File Upload' is available from the Starter plan.)
If you have already subscribed to Starter or higher but the upload window still doesn't open, please check the following: (1) Make sure the account you paid with and the account you are currently signed in to are the same. In particular, Google login and email login can be recognized as separate accounts. (2) If the accounts match but the issue persists, submit a 1:1 request with the email used at payment or the invoice attached, and a human agent will check your account status.
Note: General troubleshooting such as clearing the browser cache or trying a different browser is only relevant on a Starter+ account that is not affected by the two cases above (plan not supported or account mismatch).