How to fix not install problem after you download mod apk from HappyMod?

Method 1: Make sure you have uninstall the original one.
Have you uninstalled the original apk? because the mod apk has the same bound id with the original one, if you have installed that, the mod will can not be installed. Make sure you have uninstalled the orginal one and other version of this mod.
Method 2: Make sure you have open the permission to let your phone install the apk from HappyMod
Here is the detail toturial: