Range can't be used as a single value?

I'm an artist and only use Numbers to do merch counts and ledgers basically, so I'm definitely not very Numbers savvy. I'm sure someone here can answer this question for me pretty quickly...

I'm trying to calculate sales/profit from a convention I attended. I have my item, and sales separated by day, but I tally up totals per day at the top of my sheets so I can do a quick look/comparison. To find the total I select the numbers I want to add up and then ask it to multiply that number of units sold with the price to get my total sales. For some reason, certain ranges are okay when others are not, even though I am using the same method. Screenshots:

I haven't input much yet, but already the sales for my books and package deals are tallying up all right. I was about to add my posters when I selected that range to add to the SUM when I got this:

You can see the equation from the previous sets above. Suddenly when I add my posters I get this error:

Hovering over the exclamation gets me "(This) range can't be used as a single value" which I don't understand because it's the same method I used for the previous ones.

I've got another convention soon and I want to figure this out. If I individually add each item separately, multiply it by the price, and add it in then it works, but selecting all of them and adding 10 (in the case of the posters) will not work.

I am sure this is super basic, but I too am a pretty basic individual and I don't know where I went wrong. Would some kind soul help me get this to work so I don't have to add every single item into the equation individually?