How to fix can't install app error 505 on a rooted device?

I am on dirty unicorns 10.6 marshmallow rom. First I installed Gboard - Google keyboard app as system app through Lucky Patcher app. After restarting device I tried to open it through Lucky Patcher apps (There was no icon of Gboard app in my launcher) Launch App option but it didn't opened! So I uninstalled Gboard through Lucky Patcher. Then again I tried to install it normally through Play Store but I get this error 505 can't install app !! So there must be Gboard apps leftover files conflicting the apps installation now right ? So how can I fix this ? How can I install Gboard app now ? Note: I can install other apps without any problem !!!