You would say this Duolingo...
What's wrong with "exercise is important"? (English is not my native language, it's Dutch)
Why not øve rather than øvelse
In that case it would be "At øve er...", øve is a verb.
Why we use vigtig in sentences like ''Det er en vigtig øvelse'' or ''Så vigtig er øvelse for dig'', while we use vigtigt in ''Øvelse er vigtigt'' ?
My Danish wife says that the -t form is used when the subject refers to a general concept, as opposed to a specific instance of practice. For example, you would say "Den første øvelse er vigtig", but "Øvelse [as a life practice] er vigtigt".