Hello, there are three reasons.

1. Software signatures are inconsistent. This usually happens when the software is updated. For example, if you have previously installed an old version of a software, and the signature of the software you need to update is not the same as that of the software you previously installed, you cannot install the application.

2. The software is unsigned. This usually happens when you extract the software from the rom package or you modify it yourself without re-signing.

3. The system version required by the application is different from your mobile phone. For example, a mobile phone software requires Android system to be above Android2.3, but your mobile phone Android system is Android2.2, and the application is not installed.