Cannot click evaluation during learning

Ensure you are reviewing new cards, not those already completed. Open the navigation bar in the lower-left corner. If the bar is dark gray, the card has already been reviewed. You need to find the first card with a light gray bar, which means the card has yet to be reviewed. These cards can be found by opening the table of contents in the lower-right corner and searching for an unfinished section (e.g., with progress 39/50). Then, you can continue learning.