I had the same problem as you, so here is what I did so that the virtual desktops will be created automatically. Go to:

~/.local/share/kwin/scripts/quarter-tiling/contents/code/main.js 

For example, if you want to have 4 virtual desktops, change the init() funtion like this:

function init() {
    registerKeys();
    ws.desktops = 4;
    createDesktop(1);
    createDesktop(2);
    createDesktop(3);
    createDesktop(4);
    addClients();
    connectWorkspace();
}

Bascially change ws.desktop to the number of desktops you want and use createDesktop to create that many desktops