The main reasons are as follows: 1. No appleid: If you download it from the app store of your mobile phone or itunes of your computer, you need to enter your appleid before you can install it. Otherwise, you cannot download and install it. If you do not have an ID, please register one first. 2. Unable to install through third-party software download: If the software downloaded from the third-party assistant software cannot be installed, it is recommended to re-download and install it from the AppStore on the mobile phone or the iTunesstore on the computer. 3. There is a problem with the installation package: for example, it is incomplete or the signature is inconsistent. In this case, you can try to download it again and then install it. 4. The mobile phone storage space is full: clean up the mobile phone memory, delete uncommon software and try installation again.