Hi @Marvin_Humphrey,
I replied to someone asking about Unbounce in the Forum Post below. Unfortunately, they did not reply back about the Service Account Key.
In their documentation it says the following below.
When in your Infusionsoft account, be sure to select the option to Generate Legacy Key. Though a Service account key works the same as a Legacy Key, you can only use one at a time.
What you can do is to generate a Service Account Key, and replace the Legacy API Key in the Account.
If it works all okay, then you know Unbounce has made the changes.
Tell us if that works.