"I have pain in my neck."
Translation:J'ai une douleur dans le cou.
In English we use possessive adjectives in front of body parts (my, your, his, etc.), but in French a definite article is typically used when the owner of that body part is obvious (le, la, l'). J'ai une douleur dans le cou. Clearly I have a pain in my own neck and not someone else's so the definite article "le" is most appropriate here. You will typically find this construct with the verb avoir.
Here is a link with more info: https://french.kwiziq.com/revision/grammar/using-le-la-les-with-body-parts-and-clothing-definite-articles
Pain or douleur is a feminine noun in French, and so it requires an article. In French this is a specific, singular pain in the neck. To describe more general pain, you can use "la douleur."
I couldn't find any reference materials, but here is a good example of "douleur" being used in real life.
Sorry for the delayed response. J'ai mal au cou and j'ai mal dans le cou for "my neck hurts" are additional ways to express the same idea. From my experience, it is more common to use à la/au + body part rather than dans la/le + body part, but both are valid.