The main reasons are as follows: 1, no appleid: If you download from the appstore of your mobile phone or itunes of your computer, you need to enter an appleid to install it, otherwise you cannot download and install it. If you don't have an id, please register one first. 2. Unable to download and install through the third-party software: If the software downloaded from the third-party assistant software cannot be installed, it is recommended to download and install it again with the AppStore of the mobile phone or the itunesstore of the computer. 3. There is something wrong with the installation package: for example, it is incomplete or the signature is inconsistent. At this time, you can try to download again and then install. 4. The storage space of the mobile phone is full: empty the memory of the mobile phone, delete the software that is not commonly used and try to install again.