The tutor draws a full keyboard on screen, so it needs at least 560 × 624 pixels of page. This window has .
Try maximising the window, or pressing Ctrl and 0 together to reset zoom to 100%. It starts by itself as soon as there is room — no need to reload.
On a phone or tablet there is no fix: this teaches where the syllabics sit on a physical keyboard, so it needs one. Nothing to install — come back on a laptop or desktop.