Many have had this problem and this was settled when we kitkat but there is a method that works for them and I hope vi. I do not use because I do not know why if I can not install app and I get this error but hey if I worked Talk About
I hope that works for them as ami!
1. Download File Explorer is the Google Play Store.
2. Click the radio button shortcut menu in the upper left of File Explorer's corner (it looks like a globe with a picture of a phone in front of it)
3. When the menu is exited click the Setup icon on the bottom right.
4. Scroll down to the tool settings and click Application Manager.
5. Go to the last option and check "(un) install apk automatically"
It then will do the leg work for you when you want to install an apk, all you have to do is go to the folder where it was downloaded and click the APK file.
I think this option is much easier than other solutions involving the use of google drive or move the apk to a root folder.
You will need to activate the root explorer file explorer is for this to work, but if you need this workaround is likely to have a boot loader unlocked and rooted. To enable root explorer, click the button shortcut menu in the upper left corner, scroll down to Root Explorer, click on and allow root access.