Thank you for posting your question here. Below is the steps, this might not work for you depending on the phone you are using.
1. Go to Settings -> Language & input -> Text to speech output -> Google text to speech engine settings.
2. If you want to go on with the download, it may be that 'download over Wi-Fi only' is keeping you from doing it. Unchecked that option; switch on mobile data and your download shall complete.
3. If it still does not complete or you don't want to download at all, go to the 'install voice data' section -> select the language that the system is trying to download -> when it starts press the 'X' button on the right side.
Also unchecked the auto download option to prevent any further download.