I have had this working in the past. In my current case I hadn't set the track as a midi track before the generation though and I thought that might be the case. So I went back again, set the track as midi, selected a midi style that included the acoustic guitar. Selected Generate Midi, but again got the message the style did not have a guitar part. I tried the string part for the same style and that generated OK.

The style selected was SERENITY - Serenity - Slow 16th Pop Ballad

I can generate Strings and Drums only. The other tracks report that the instrument is not in the style.