In order to understand and assist, we would need you to attach the specific symphonic score you are having problems with. Assuming other scores play, most like the instrument settings are simply incorrect for this one and should be corrected in View / Mixer, or you might be missing a soundfont that particular score relies on.
If you are having trouble with all scores - including simply entering a note into the default empty score - see https://musescore.org/en/node/96811 and carefully follow each of the suggestions there.
Comments
In order to understand and assist, we would need you to attach the specific symphonic score you are having problems with. Assuming other scores play, most like the instrument settings are simply incorrect for this one and should be corrected in View / Mixer, or you might be missing a soundfont that particular score relies on.
If you are having trouble with all scores - including simply entering a note into the default empty score - see https://musescore.org/en/node/96811 and carefully follow each of the suggestions there.