Add a variant to the cart, not the product itself.
When trying to add an item to the cart I was getting the following error
41 10:23:28 ERROR CtxMsg.Error.EntityNotFound: Text=Entity 'Catalog|Modem|' was not found. 41 10:23:28 ERROR PipelineAbort:Entity 'Catalog|Modem|' was not found. 41 10:23:28 ERROR PipelineAbort:Entity 'Catalog|Modem|' was not found. 41 10:23:28 DEBUG Executing ObjectResult, writing value "Microsoft.AspNetCore.Mvc.ControllerContext". 41 10:23:28 DEBUG Executed action "Sitecore.Commerce.Plugin.Carts.CommandsController.AddCartLine (Sitecore.Commerce.Plugin.Carts)" in 107.9314ms 41 10:23:28 DEBUG Request finished in 109.1548ms 200 application/json; odata.metadata=minimal; odata.streaming=true; charset=utf-8
This was very confusing as I could clearly see that the product existed in the Commerce Server Catalog Manager and there were no variants configured. I was having a WTF moment and illustrating symptoms of insanity by making the request, again and again, expecting a different result.
After working through the setup of the product for the fifth time I noticed I had some properties defined for the variant of the product. See the screenshot below from the Commerce Server Catalog and Inventory Schema Manager
Seeing these properties I wondered if this was the underlying cause of my issue and removed them. BAM!! I was now able to add the product to my cart!