The final level in the game is not available until you have found the "perfect" solution for every other level in the game (no more yellow blocks in the level overview).

To "perfect" a level means to complete it using the minimum number of moves.
https://gtm.steamproxy.vip/app/551110/discussions/0/2592234299545690312/

Completing the final level also gives access to the 10 bonus levels.
https://gtm.steamproxy.vip/app/551110/discussions/0/1693795812290067462/