To be able to digivolve, your digimon need to be at certain level . Here is the list :
Champion Stage : Lv 11
Ultimate Stage : Lv 25
Mega Stage : Lv 41
Burst Mode : Lv 65 and certain condition
The picture below is the example, you need to be at lv 11 to open the champion stage.
When you meet the condition, you can digivolve only by click the evolution stage picture.