Learn German: Der Streit ist nicht romantisch. - The argument is not romantic.

The noun der Streit (the argument) comes directly from the verb streiten (to argue). You will often see German nouns built this way from the verb root.

If I want to ask 'Who are you having an argument with?', how do I use the question words from this lesson?

To ask who someone is arguing with, you use the verb streiten with the preposition mit (with). Since mit always requires the dative case, you use wem (whom) for a person: Mit wem streitest du? (Who are you arguing with?).

Why do we use nicht here instead of kein?

We use nicht because it negates the adjective romantisch. You only use kein to negate nouns that lack a definite article. For example, to say 'That is not an argument' you would say Das ist kein Streit.

Does der Streit refer to a physical fight or a verbal argument?

Der Streit almost always means a verbal argument or dispute. If you are talking about a physical fight, you would normally use the noun der Kampf.