Trill Intervals

Do you mean you &want* the trill to be between G and F? If so, you need to write the note as F, not G. Trills are always to a note above.

if that doesn't solve the problem, please psot the actual score and tell us exactly where it isn't working as you would like and how you would like it to be instead.

Be aware, though, that the primary purpsoe of MsueScore is notation, not playback. Some things just won't play back automatically exactly the way real musicians would play it. If it is important to you to have more control over the computer playback, you can instead enter the notes manually, perhaps on a separate invisible staff.