Target API level requirements from August 2018
Your app currently targets API level 21 and must target at least API level 26 to ensure it is built on the latest APIs optimized for security and performance.
What is it means...?
your app needs to be updated sdk to 26 or above.
check your minsdk.
check this for more explanation
i just entered manually 26 into config and built with cocoon... no warnings
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Thank you I learnt how to edit config file thanks all of you