"Ducks are birds."
Translation:Enten sind Vögel.
⟨t⟩ is pronounced, but only as a glottal stop. I don't know what your native language is, but if it is English (or you know it well enough), the pronunciation of ⟨t⟩ is the same that most (all?) English varieties use before a nasal consonant (m, n)—for example in ‘button’—or that Cockney uses in most positions. German does this to some degree (depending also on speed and carefulness of enunciation) with all stops (t, d, p, b, k, g) when preceding ⟨(e)n⟩. The place of articulation of the stops dictates the realisation of the nasal consonant (⟨t, d⟩ → [n]; ⟨p, b⟩ → [m]; ⟨k, g⟩ → [ŋ]).
depending on your tablet and the keyboard you're using, often you can hold the key to get to accented versions, umlauts, etc.
but even if that doesn't work, you should also be able to substitute any umlauted letter with the plain letter + 'e', so for example if you can't make Vögel, Voegel is equally acceptable---not only in Duolingo, but in German writing broadly.
(Note: this is only true for German; Swedish and other languages that use the same characters have different rules for replacing them.)