> There’s a chord in the piano bass composed of the notes D, A and C.

What's in the melody? That will probably give you a clue to what the harmony is meant to be.

I just reread your posting and I think you were asking about the chord buttons and not just the single note ones. There is no combination of chord buttons you can use to get this combination of notes. They all have 3 notes unless you have buttons for the diminished 7ths which have 4 (I believe --- my accordeon doesn't have them). The triads have what you would expect, except not in root position, and the dom. sevenths are missing the 5th, which makes it possible to play 7ths with flat or augmented fifths (with the latter in the discant).