بالدفتر في صفحة ناقصة.

It means in the notebook.

It is made of:

• بـ = in / inside / with depending on context
• الدفتر = the notebook

So بالدفتر is literally in the notebook.

In Levantine pronunciation, this is often said something like bid-daftar, because the د causes the l of the article to assimilate in speech.

Yes, في can mean in, but here it is being used in another very common way: there is / there are.

So in this sentence:

• بالدفتر = in the notebook
• في = there is
• صفحة ناقصة = a missing page

So the whole structure is basically:

In the notebook, there is a missing page.

Because they are doing two different jobs:

• بالدفتر gives the location
• في introduces existence

A very literal breakdown would be:

In-the-notebook there-is a missing page.

This is very natural in spoken Levantine.

Because Arabic often does not use a separate present-tense verb to be the way English does.

In English, you say:

There is a missing page.

In Levantine, في often covers that there is meaning by itself, so no extra is is needed.

Because صفحة is a feminine singular noun, and adjectives must agree with the noun.

So:

• ناقص = masculine singular
• ناقصة = feminine singular

Since صفحة is feminine, you need ناقصة.

Because the sentence is introducing a page, not referring to a specific already-known page.

After existential في meaning there is, Arabic very often uses an indefinite noun:

• في صفحة ناقصة = there is a missing page
• في الصفحة الناقصة would sound more like there is the missing page, which is a different idea and usually would need a different context

So the indefinite form is the natural one here.

Here it most naturally means missing.

But ناقص / ناقصة can have a few related meanings depending on context, such as:

• missing
• lacking
• incomplete
• short by

In this sentence, with page, the intended meaning is most likely missing.

Yes, you could.

For example:

في الدفتر صفحة ناقصة

also makes sense and is very natural.

Using بالدفتر is also natural in Levantine speech. One reason speakers may like بالدفتر here is that the sentence already contains في as there is, so بالدفتر helps avoid repeating في twice in a row.

So both are possible, but the given version sounds nicely colloquial.

Because Arabic often puts the setting or location first.

So instead of starting with there is, the sentence starts with in the notebook, then tells you what exists there.

This word order is very common and natural in Arabic:

بالدفتر في صفحة ناقصة
= In the notebook, there is a missing page.

A common pronunciation would be:

bid-daftar fī safḥa nāʔṣa

A few notes:

• بالدفتر → often sounds like bid-daftar
• صفحة → often safḥa
• ناقصة → in many Levantine accents, ق becomes a glottal stop, so it may sound like nāʔṣa

Exact pronunciation varies by country and city, but that pronunciation is a good Levantine-style guide.

Yes, it feels more like spoken Levantine.

Why?

• بالدفتر is very colloquial in this kind of sentence
• using في as there is is a major feature of spoken Arabic, including Levantine
• the overall structure sounds natural in everyday speech

In Standard Arabic, you might more often see something like:

في الدفتر صفحة ناقصة
or
توجد صفحة ناقصة في الدفتر

But the sentence you were given is a good spoken Levantine-style sentence.