Engagement Rate divides the sum of [likes, comments + saves] on your posts by the number of followers that you had at the time of the post.
Engagement on Reach divides the sum of [likes, comments + saves] on your posts (including videos) by the amount of Reach that your post received, ie. how many people saw your post.
Simply click on the little 'ER' icon in the top right to switch the chart.
You'll also find both these metrics in your post-by-post data: