This is because Sololearn works on servers, and they have very less memory assigned per user, and so, you cannot modify much of data on it.. snd when you try to download something inside Sololearn, it won't work.
Its not at all possible in Sololearn
I tried it with a Google logo image and it seems that SoloLearn doesn't support download. What I did and you could do is upload it to somewhere like imbb.com, and embed it into the href and download attributes like so:
<a href="https://i.bb.co/......" download="https://i.bb.co/....." target="_blank">DOWNLOAD Button</a>
This means if download isn't supported then SoloLearn will redirect the user to open a browser (app) or open a new tab (SoloLearn web) to see the image and download it themselves. It's also good for browsers that don't support the download attribute (which I believe IE should be blamed as well).