Because you downloaded this software before, he uninstalled it himself, but it was not clean. His software signing certificate is still there, which caused a conflict when you reinstalled it. That's why the monitor can't be installed.
Try double erasing (Baidu Encyclopedia is very detailed) to clear the data of the mobile phone. So you can download the software. You can download it through the application treasure, which contains a lot of software.
I hope I can help you. Take it.