Manca una virgoletta in quella frase.

Because the grammatical subject is una virgoletta, which is singular.

In Italian, mancare agrees with the thing that is missing:

• Manca una virgoletta = one quotation mark is missing
• Mancano due virgolette = two quotation marks are missing

So even though una virgoletta comes after the verb, it is still the subject.

Yes. Italian often puts the subject after the verb, especially when introducing something that is missing, present, needed, and so on.

So:

• Manca una virgoletta
  literally works like A quotation mark is missing

This is very natural in Italian. You could also reorder it:

• Una virgoletta manca in quella frase

but that sounds less natural in most contexts.

Here mancare means to be missing, to be lacking, or to be absent.

That is an important pattern in Italian. English speakers often first learn mancare through sentences like Mi manchi, where it feels like to miss someone. But the core idea is still that something is absent.

In this sentence:

• Manca una virgoletta = A quotation mark is missing

So the focus is on what is absent, not on a person doing the missing.

Virgoletta means quotation mark.

A few related words are useful:

• virgola = comma
• virgolette = quotation marks, inverted commas
• una virgoletta = one quotation mark

In practice, virgolette is very common when talking about quotation marks as a pair, but una virgoletta is perfectly normal if only one mark is missing.

Because Italian normally uses an article here, and the speaker is referring to a quotation mark, not a specific already-identified one.

So:

• una virgoletta = a quotation mark
• la virgoletta = the quotation mark

If someone is just pointing out an error in a sentence, una virgoletta is the natural choice.

In quella frase means in that sentence.

The preposition in is used because the quotation mark is missing within the sentence, as part of its written form.

So the structure is:

• manca = is missing
• una virgoletta = a quotation mark
• in quella frase = in that sentence

You could think of it as locating the problem inside the sentence.

Quella is a demonstrative adjective meaning that. It agrees with frase, which is feminine singular.

So:

• questa frase = this sentence
• quella frase = that sentence

Because frase is feminine singular, the form is quella, not quello or quel.

Yes, frase can mean sentence in many everyday contexts.

This can be confusing because it looks like English phrase, but in Italian it often covers what English would call a full sentence too.

So in a context like grammar, proofreading, or punctuation:

• quella frase often means that sentence

Yes. Italian is fairly flexible with word order.

These are possible:

• Manca una virgoletta in quella frase
• In quella frase manca una virgoletta

Both are natural. The second version puts a little more emphasis on in that sentence.

The original order is completely normal and straightforward.

You would make both the noun and the verb plural:

• Mancano due virgolette in quella frase = Two quotation marks are missing in that sentence

Notice the changes:

• manca → mancano
• una virgoletta → due virgolette

That is a very useful pattern to remember with mancare.

It is neutral and natural. You could use it in:

• proofreading
• editing
• class
• everyday conversation about writing

It sounds like standard Italian, neither especially formal nor especially casual.