Google Play Store - Cannot Install Beta

Question

I am developing a new version of my application for Android, and I would like to distribute a beta version for my testers. I created a group of private testers, and then I downloaded my beta version of the APK (over 24 hours ago). At the moment, I have come across strange behavior:

• The Google Play Store has notified my app update for my testers only;
• When they try to update, they also receive errors such as: "The application cannot be installed because it is still installed. Error code: -1"

I signed the APK with the same keystore I used before and installed a higher version of versionCodeand versionName. The application package is still the same.