The answer is in the link provided by Taifun Andreas... Android 8.0 Oreo and later
Google changed how APK installations work in Android 8.0 Oreo. What you see is not a bug; it is App Inventor reacting to Google's requirements for apps on Android 8+ .
You can download the apk to your PC; upload the apk to GoogleDrive as shown here> Give an app to a friend - #5 by SteveJG or several other ways. The link Taifun posted says you don't have to use Chrome...if you download to your PC ... Android will ask you to grant permission to either the file browser or your web browser to install the app. You have a choice.
Google Chrome does not randomly install unknown apps because of security risk while surfing around. You have to have the install from unknown sources setting set. If it is not set, you won't get unwanted stuff. Set unknown apps when you download an App Inventor app and if you are uncomfortable; un set unknown apps.