Cannot be missed if you are playing through the 'story' mode.
Will pop after the final Gravely Grove level. Some clarity from the comments:
In case anyone else was wondering Gravely Grove is the 5th stage/world in the game 😊
1. Peggle Institute
2. Mountain Highs
3. Winter Blunderland
4. Gnorman's Gneighborhood
5. Gravely Grove
6. Celestial Realm