This happen when the app that you are testing in-app updates is not signed with the same signing key as the one available from Google Play.The reason is android OS is not able to install the update as the app that you are testing is not signed.(ie using the debug version)
Generate a signed apk using the same signing key(key used to sign the app uploaded to Google Play) and install it in your device. This will download and install the update.
| Generate a signed apk | Use the same signing key |
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