All Android app before publishing require digitally signed with a certificate, anroid system is not the installer does not sign the (security considerations, can find related documents)
The signing process details, see: https://www.cnblogs.com/a418120186/p/5475621.html
The following summarizes some key points:
1. Generate a signed certificate
2. Set build.json
3. Generate Keystore into the project root directory cordova
4. The error may be generated
Modify the file: \ Learun.Application.Mobile \ platforms \ android \ build.gradle
The final generation: