You need to reach level 12 to farm. The cultivation tab shows progress, the more you cultivate the other summons, the greater the progress, leveling up you release new summons.

Hello,
In the little box at the screen you are currently highlighting that shows the tortoise, you can see that it says that it requires Summoning Jutsu level 12. At the top of the screen you can see your current summoning jutsu level (level 10). It is just behind lightning and fire forces.

You will need to cultivate enough other summons until you reach level 12 to progress this summon.

Thanks and have a great day!