When you click the Soundtrap link, you will find a performance of "Mary Had a Little Lamb."
You will hear that some of the notes are off in rhythm. Fix these by quantizing. Try different settings (1/2, 1/4, 1/8, 1/16) until it sound correct in its timing.
There are also a wrong notes. Double click the track (Red Rectangle) and you will see the piano roll. Play the song again until you find the wrong notes. Move the notes up or down to make it the right note(s) (The notes of the song should only be C, D, E, and G).
Once you have fixed the song, save it then download and attach an Mp3 of the project to the Google Classroom assignment. I made a tutorial video on how to do that here: https://bit.ly/2FSpJJQ
If you need to review how to quantize or fix notes on the piano roll, go to this link: https://sites.google.com/egs.edu/mr-burba/units/unit-2-structure/7-quantization