Sometimes it is not easy to tell what images the automated system is asking about so it may be best to hit the “Save & Close” button here and check/fix image alt-text issues from the “Accessibility Report” where it is easier to see the images since they are highlighted. (See section III)
Video walking you through the steps to run Adobe Acrobat Pro DC's "Accessibility Checker" on your PDF.
Checking a PDF's reading order is very important to ensure accessibility. Here is a video walking you through checking your PDF's reading order using Adobe Acrobat Pro's "Accessibility Checker".