You can find how to activate "dark mode" on your iPad, iPhone or iPod touch here: Apple Support
There are a few things we can check:
Ensure that you have given Symbol-It permission to use both the microphone and speech recognition. You can check this in the Settings App under Privacy → Microphone and Privacy → Speech Recognition.
Symbol-It can't translate what you're saying into symbols unless Siri is enabled for your device. To turn on Siri, you need to enable one of the three following settings in the Settings App:
For a live demonstration of this fix, please see this YouTube video.