Musescore 3 problems

Without looking at your computer it's not possible to do more than tell you what you posted in this situation. According to what you say you are only using the standard sound font. When you do this, you do not have the opportunity to upload score audio. You also have told us that you have uploaded the score once already to musescore.com since you have the option to update the score.

To trick the audio you need at least two soundfonts loaded. There are then two possible ways to force the program to allow you to upload audio. First, make the default sound font second in the list and manually set all of the sounds in the mixer. The second option is to keep the default soundfont first which will automatically set all of the sounds to that font but set at least one instrument's sound to a different soundfont. You will then be given the option to upload the audio with the score.