can't upload the apk

We found that sometimes the iFrame will display this message instead of the proper error, is this the first publication of your app ?

If so have you checked that it's properly signed with a release signature and that debug mode is disabled?

If you already have a version present on the Google Play, make sure that you've increased the version of your new build, and check that you have not changed the application ID (package name).

You can always access your Play Store account directly from the Play Developer Console and try to upload your app from the Developer Console where you should get a better understanding of the issue.