In the Text-to-speech settings window:
Pay attention!
The preferred engine should be selected as “Speech Services by Google”
Pay attention!
The setting “Use Wi-Fi only” should be disabled.
It allows to download new voices when you don’t have a Wi-Fi connection.
This setting is enabled by default on many devices and causes the problem with text-to-speech voices.
After all those suggestions, the text-to-speech should work. If not, please contact us directly from the app per email: feedback@lexilize.com, per WhatsApp or Telegram.