You may want to post your question on StackOverflow and tag it with the "firebase" and "dialogflow-es" tags with as much information as you can. Sample code will help a
lot, as well as an explanation of what you're trying to do and what tutorials you've been following. "Connecting dialogflow to Firebase" is a pretty broad statement, so being as clear and specific as you can will go a along way.
The broad answer to your question, however, is that you need to set the FIREBASE_CONFIG environment variable, as the error states.How you do that depends on a lot of things about how you're trying to access the database, which is why showing code helps.
Allen