If Amazon ads are not displaying on single post, try to disable AdBlocker on your web browser.
Sometimes for the default keywords there are no Amazon products to be shown due to Amazon product stocks fluctuate daily.
You can try to customize the ads code with different keywords from WP Automatic -> Settings -> Ads Option: http://prnt.sc/rtbv1q
On Top Ad Code options, find this code:
amzn_assoc_default_search_phrase = "mouth mask";
And replace the mouth mask with other keywords such as face mask, n95 mask or other keywords.
On Bottom Ad Code options, find this code:
amzn_assoc_default_search_phrase = "purell sanitizer";
And replace the keyword with other keywords.
You might also enhance the product search by customize this code:
amzn_assoc_default_category = "HealthPersonalCare";
Replace HealthPersonalCare to All, that way it will display all products for your keywords on all categories instead of just Health category only..