I see a sentence,
Ein gesunder Körper braucht ausreichend Bewegung.
Bewegung is female, so shouldn't it be
Ein gesunder Körper braucht ausreichende Bewegung.
? Is the text on the site wrong?
Interesting case. The move test shows that the word «ausreichend» is an attribute of «Bewegung», not an adverbial phrase:
You can move the entire phrase to the Vorfeld:
The question test corroborates that they «ausreichend Bewegung» forms a single constituent:
The replacement test shows what the word ausreichend is – it is a quantifier (or quantifying particle) like «viel» or «wenig»:
ausreichend is originally the present participle of ausreichen, but today often classified as a self-standing adjective.
German wants adjectives that are attributes of substantives to follow the flexing of these substantives. In all other cases, adjectives do not follow case, number and genus of the substantive. Your example sentence can be considered as one of these non-attributive usages.
What you observe here is what is called a Depiktiv, the ausreichend is in this case not a qualifier of the substantive, but rather the predicate and is thus not flexed (You could consider this an adverbial usage).
Note your second example is not wrong, it just has a slightly different meaning.