I am getting miscalculations for the review / sales calculation in 'third chart' Tableau

I have followed the instruction to the 't' twice and am getting the incorrect answer. I have created recreated the workbook each time I have attempted. The problem is, when dividing the "Number of Ratings" by the "Number of Purchases", my values on the y axis start from zero and increment by one all the way up to six. When I check the calculations made in data source tab, all of the values are null. When I drag the 'Reviews /Sales' tab onto an empty sheet it shows a ratio of 0.10. I am not sure why I am getting these values on my y axis. Also, I am not sure how to fix these values to get the correct distribution. How do I check the data and what is my solution? Thanks