That happens sometimes. It’s a network congestion problem at store.kde.org’s end, and KDE is aware of it — see this bug report.
For now, just give it some time and then try again. 
Or, you could try another approach — I think I’ve already told you about this. Install ocs-url from the AUR, and then you can install the theme straight from the page for that theme in your browser by clicking the OCS link, as in the picture below. 
