The Sheet Music and Piano challenge in Week 6 Season 6 is split into 4 stages. First, you need to find the sheet music in a house in Pleasant Park. You have to wait until the next match until stage 2 can be completed. The piano on the hill next to pleasant park has now the sheet music standing in front of it. Play it to unlock the next stage.
It follows the same pattern. Simply pick up the sheet music in the east of retail row and play it on the piano next to retail row.