My understanding is that the definite article is required for language nouns, except with "parler", in which case it is optional (and most commonly dropped). So Duo should accept "Il parle anglais" or "Il parle l'anglais".
I had the EXACT correct answer and it marked it as wrong. I have been getting this a LOT lately. Sometimes the answer did not have an accent mark or a circonflex, but the remarks used to come back as correct (but remember to pay attention them). Now it just marks the answer as wrong. I do not know how to add those diacritical marks. This is getting REALLY frustrating. I have to quit the lesson as it just keeps going in an endless loop until you get the answer correct. There is no way to get the correct answer in these situations.
With a second person, singular subject ("tu parles").
You need to memorize the conjugations of (at least) the regular -er verbs: je parle, tu parles, il/elle/on parle, nous parlons, vous parlez, ils/elles parlent.
The root is found by dropping the "er" ("parler" -> "parl") and then you add the endings "e" / "es" / "e" / "ons" / "ez" / "ent" based on the subject.
In the masculine "anglais" (found in this sentence), the "s" is silent. In the feminine "anglaise", the "s" is pronounced as "z".
Note that in this sentence, "anglais" is a noun for the English language, and is always masculine. If used as an adjective ("English"), it can be masculine or feminine. It can also be used as a masculine or feminine noun meaning "Englishman" / "Englishwoman", in which case it is also Capitalized.
If you can, try to look at and become familiar with "conjugation (charts)" for the "irregular" verbs « être, avoir, faire, aller » and the "regular" verb endings « -er,-ir, -re ». In this case, « parlez » is the « vous » conjugation for an « -er » verb, whereas « Il » is « parle ». There are more complicated tenses later, but at this point, tiny supplements like this have helped us when the audio is a bit distorted or unclear!
Are you in the "word bank" form of the exercise, with word buttons? Are you saying that the word is there but clicking on it doesn't do anything? Is the button partially clipped? Web browser or app version of Duo? There is a known bug with the browser version where you may have to zoom out to see the whole button. If you see this again, take a screen shot and file a bug.