Google Play installer verification requires a minimum SDK version of 21 or later. The submitted app package has a minimum SDK version of 14. Update minSdkVersion in the app's Android manifest.
minimum sdk explained
If you intend to develop commercial programs and post on Google Play you really need to read and understand most of the posts you see in the links above. 
There is certainly a solution for YOUR issue.
protip: try a search in the community to find the answer
https://community.appinventor.mit.edu/search?q=min%20sdk%2021
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