Duolingo's systems ignore all punctuation marks. It interprets numbers as 'words', but commas, full stops, question marks, plus and equals symbols... all get thrown out (try adding them in between words in other sentences - it has no effect as far as I can tell). As a result, if you do that here it will think you're 'missing words'.
This whole "math" lesson should be expanded or absorbed into one of the numbers lessons. At the moment the only point is to learn four words: plus, minus, gleich, and macht. Two of these words are spelt exactly the same as their English equivalents, and the other two we know already - it's useful to know they can be used to say "=", but it shouldn't take a whole lesson just to figure that out.
Because "add" is a verb, you can't use it by itself. It needs helping adverbs and prepositions and conjunctions. For instance: "add three and four together to get seven "
Plus however is a conjunction, which combines two nouns to get a single result, and just needs an additional verb to make a sentence: "Three plus four equals seven. "