You still need to properly sign the application, disabling that checkbox does not sign the app for release to the Google Play Store. Please see Signing | PhoneGap Docs for more information on generating your signing key and the process.
Thank you for your answer. Yes, I've already generated a private key according these given instructions. As there wasn't any errors, during these steps, so, I think, all validation is passed successfully. But still I do not know how to solve a problem.
By the way, I am not sure about an alias: "...ensuring you use the same alias used when you generated your key". What exactly alias should be there? Any advice or example? http://docs.phonegap.com/phonegap-build/signing/android/#generating-a-private-key > Submitting your key to Build (after 7th Step).
The alias can be whatever you like. The documentation is just indicating that you have to give PGB the same alias or it won't be able to use the key.
As for what you might use -- you could use the app name or even "phonegap". I tend to do the former, however.