I can’t understand why it says I have to redecorate to move to the next level, it says I have enough..

To move to the next level, you need to add Flowers to your island, the amount of which is indicated in the upper right next to the flower icon. Redecorate your island is one of the ways to do that.

Flowers can be earned in several ways: buying decoration, upgrading buildings, renovating landmarks, and sending Jack's plane on a flower delivery. When you buy decoration, the construction must be complete and the red ribbon removed before its Flower value will count. Buying decoration of higher Flower value will get you further in the game.