Second Galaxy has a certain experience limit that can be gained per day.

Many questions related to the termination of experience at level 19. This is due to the fact that the player has reached the maximum experience limit.

So for example, on the first day of the game, the player can reach level 19 and 70%, then whatever the player does, he will not receive level 20. You need to wait for the next game day and play on. The quest list shows how much the limit is reset.

Second Galaxy Daily Experience Limit