As per the description above, you are not able to use the Talk to Text Feature in Adobe Fill and Sign, Is that correct?
I tried using the Adobe Fill and Sign App with the Voice Typing i.e. Talk to Text in my Samsung S8 Plus and found that by default the Samsung Keyboard was selected, and Google Voice typing was missing.
So I tried following steps:
Make sure you have the Google Keyboard installed from Google Play Store.
Hope this helps, and if the issue persists, please share the Model number of Samsung Phone you are using so that we can assist you further.