Hi! So to cultivate a summon, you need to meet a few criteria. Having the summon is the first criteria (which you have!), the next criteria is to meet the required Summoning Jutsu Level.

For example:

I have the summon Guy's Tortoise , but I need the required Summoning Jutsu Level of 12 (You can find your Summoning Jutsu Level on the top of the "Cultivation" Tab) in order to cultivate it.

My Summoning Jutsu Level is 11, therefore, I cannot cultivate it.

~Jib