"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...
That is true, but it's not the reason in this case. Laufen would move to the end for the same reason verbs move to the end when used in conjunction with verbs like werden, haben, seid, etc. The first verb is always in the second position and is conjugated according to the subject, and any other verbs are moved to the end and left in their infinitive.