There are a few reasons why this error would occur.
- If your current plan has expired, then all premium features will be disabled.
- If you are on a Basic plan but are using a Pro voice.
- If your payment method has failed, due to which your plan has not been renewed.
- If an invalid IPA has been added to a particular word.
- If an unrecognized character is present in your script. (Recommend: Paste text as plain text using Ctrl/Cmd + Shift + V)