"I go running very often."
Translation:Ich gehe sehr oft laufen.
Verbs for running don't really translate 1:1 between English and German - a source of constant confusion until you get a feeling for it through exposure to native-speakers.
rennen really emphasises running fast, often in a race. Somewhat like "sprinting" but again not an exact match. If you used this word, people would likely ask why, rather than just understanding that you run for fitness. That would be more commonly expressed with laufen (or more specifically joggen for "jogging").
So, whilst not wrong, maybe Duolingo doesn't accept it because it would be a less usual way to say it. Or, maybe they would accept it if you reported it as a missing alternative...