Can't upload to internal storage

I've just got a "new" (hand-me-down) phone, about 2 years old.
It's a Chinese brand, iTouch, Airdroid IDs it as "AlpsHD88+, running Android 4.14.

It has 6 GB internal storage, 4GB free and no SD card as yet.
Airdroid also shows it having 460 MB of SD storage, zero free. As there is no SD, I assume this is some kind of virtual disk?

I don't need massive storage and just want to load some ebooks onto it to read, each a few hundred kB.

But when I connect with Airdroid I can't copy from the PC.

I try to create a folder in the system/data directory, but get "Failed to create folder".
Nothing happens at all if I try to copy files from the PC.
If I try to copy to the "SD card" it says "Failed. Not enough free space on SD card".
(As I said, there is no SD card.)

So is it possible to use the 4GB storage I have or must I use an SD card?
I assume it's a permisisons problem. Is it fixable?
 

You are using web.airdroid.com? Can you attach some screenshots of it? The SD Card is a defult folder in your phone? Or you created it on web.airdroid.com?

Why can't I get it working

Why can't I get it working 

Can you tell me more details of the issue?