It sounds like you are trying to start it from phone app. It is better to record it from the watch itself using the Exercise App. From the main clock face, scroll up (swipe down) to 'Exercise'; tap that and scroll up or down to find the desired exercise category. If HIIT, or anything else, is not in the list, the first 2 topics in the following 'Help' section describe how to add it as an exercise shortcut:
How do I track my workouts with my Fitbit device?
Before posting, re-read to see if it would make sense to someone else not looking at your Fitbit or phone.
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