Re: not enough memory to install ap
I'm not familiar with this phone, so I did a little searching for their specs. It looks like they run on the low (if not obsolete, by today's standards) side of specs. As Rukbat said, you first need the internal space to initially install any app, and some of it will still remain on the internal storage even when it has been "moved to external sd." It looks like the Cubot phones I saw were all 4 GB internal memory. A large chunk of that will be taken up by the operating system and pre-installed apps. That leaves very little to install additional apps to begin with. Today's apps are getting a lot larger as well, so only one or two could suck up enough internal storage that you start getting low space warnings.
Unfortunately, I think your only real solution is to get a newer phone with more internal space to begin with. 32 GB is adequate for most people today, but more is always better. There's 16 GB offerings still too, but even those will start suffering if you're not careful with space usage.