How to solve "application can not be installed" error I am facing an application can not be installed error on my Samsaung Galaxy S Plus whenever I try to install applications. I have already allowed installation from unknown sources, but it didn't work. It still have a lot of memory space although my phone has no SD card. 

Can you suggest some solutions to solve this? Thanks in advance.
First, make sure you have sufficient internal memory (that should be more than just the app size, because app needs to store data too). The storage for apps is different from internal mass storage. You can find it under Settings > Storage, look for the Applications label.

Also, if the app is, say 20 MB, keep at least 50 MB free, because apart from the app storage, Android also tends to slow down when short on internal memory.

If you're trying to update/overwrite an installed app, try removing it first. source.

If that didn't work, try this solution. It is aimed at Titanium Backup, but I guess it might work for other apps too.