Chances are you don't have enough points to select all of your images. It should tell you how many images you have enough points to process at the top of step 4.
The Mavic Air 2 has kind of goofy images. There is basically no reason to use them at the 48 megapixel resolution. Just use the 12 MP setting. With the 48 MP setting you can only select 1/4 the images and it really doesn't help the level of detail.
Being that we recommend having overlap that corresponds to 15-20 looks at everything there is a minimum of 20 images required for each job. The blue button should be telling you that you need to select 5 more.