"I like running very much."
Translation:Ich laufe sehr gern.
Please see Max.Em's helpful comment, which explains how Germans really use these words. Often, even if there's a word that sounds similar to English ("run" & rennen) it's not the best translation.
That said, it seems the database should have actually accepted your answer. I think I fixed the problem now.
But again, just because Duolingo accepts it, it doesn't mean it's the best answer. I can highly recommend paying attention when native speakers offer their advice about word usage :)
Running as a sports or leisure activity is laufen, running as a way to reach something fast is rennen. Only children could say "Ich renne sehr gern", while I don't know if this would be "I like running very much" in English. So "rennen" is probably not incorrect here, but very rare compared to laufen.
Yes, this is also laufen, not as a leisure activity, but as a way to arrive somewhere. In some cases it's not so easy to distinguish, for example when I say "Ich laufe (sehr) gern" this could be running (as a sport) or walking (to some destination, e.g. the job or supermarket) as well.
If it's not clear by context (Nehmen wir den Bus oder laufen wir? vs. Ich laufe jeden Morgen im Park), you can say "Ich gehe gern laufen" (sport) or add a destination, e.g. "Ich laufe zum Supermarkt". As the latter can still be interpreted as running, you could switch to "ich gehe zu Fuß", which is totally unambiguous.