Thank you for participating in the Watch 4 Beta and for your feedback!
Within Samsung Messages app on the Watch, in the Message composer window, please tap on the text box. Once you tap on that, a keyboard will be displayed. If you swipe up on the keyboard, you will see options for different keyboard settings, as well as a 'microphone" icon to turn on voice to text. Once you set the permissions to "allow", you will be able to use the voice to text function.
If the error persists, please let us know.
One UI Beta Team