In Sibelius , I can't export my score as a mp3 file. Because when I'm in export page, I choose my playback configuration that is connected to my digital piano. But it says "you can't use this playback configuration for exporting audio because it does not contain any virtual instruments please choose another playback configuration above" What should I do?
A 'virtual instrument' means Sibelius 7 Sounds or one of the other software playback systems that can be run within Sibelius. It has no way of 'hearing' the sounds your external keyboard makes, so cannot directly Export an MP3.
If you particularly want the sounds of your external keyboard, record its audio output. The recorder COULD be another program running on your computer, with the audio out of the keyboard connected to your soundcard inputs. Or it could be a seperate external recorder.