Esta mensagem é urgente, porque falta uma assinatura no formulário.

Yes, you could often hear Essa mensagem in Brazilian Portuguese.

In more traditional grammar:

• esta = this, something close to the speaker or the present context
• essa = something close to the listener, or something already mentioned

In real-life Brazilian Portuguese, though, the distinction is often weaker, and essa frequently replaces esta in everyday speech.

So:

• Esta mensagem é urgente = very natural, especially in formal writing
• Essa mensagem é urgente = also very common in Brazil

Because the sentence is referring to the present message itself, esta mensagem is a very natural formal choice.

Because mensagem is a feminine noun in Portuguese.

Words that go with the noun usually have to match its gender and number:

• esta mensagem
• a mensagem
• uma mensagem

If the noun were masculine, you would use masculine forms:

• este formulário
• o formulário
• um formulário

So the form esta is used because mensagem is feminine singular.

Because urgente is an adjective that has the same form for masculine and feminine in the singular.

Examples:

• uma mensagem urgente
• um aviso urgente

In the plural, it changes normally:

• mensagens urgentes
• avisos urgentes

So urgente does agree, but its masculine and feminine singular forms happen to look the same.

Because é is the verb ser in the present tense:

• é = is

The accent helps distinguish it from:

• e = and

So:

• A mensagem é urgente = The message is urgent
• A mensagem e o formulário = The message and the form

That accent is very important.

Here, porque means because. It introduces the reason:

• Esta mensagem é urgente, porque falta uma assinatura no formulário.
• The message is urgent, because a signature is missing on the form.

This is one of several similar-looking forms:

• porque = because
• por que = why / for what reason
• por quê = why, when it comes at the end
• o porquê = the reason

Examples:

• Por que você saiu? = Why did you leave?
• Você saiu por quê? = You left why?
• Não saí porque estava cansado. = I didn’t leave because I was tired.
• Não entendi o porquê. = I didn’t understand the reason.

In your sentence, porque is the correct form because it means because.

The verb faltar is very commonly used to mean:

• to be missing
• to be lacking

So falta uma assinatura literally works like:

• a signature is lacking
• more natural English: a signature is missing

This is a very normal Portuguese structure.

Compare:

• Falta uma página. = A page is missing.
• Faltam duas páginas. = Two pages are missing.
• Falta seu nome aqui. = Your name is missing here.

You could also hear:

• Está faltando uma assinatura no formulário.

That is also very natural in Brazilian Portuguese and sounds a bit more conversational.

The grammatical subject is uma assinatura.

That may feel backwards to an English speaker because the verb comes first, but Portuguese often does this.

So:

• falta uma assinatura = one signature is missing
• faltam duas assinaturas = two signatures are missing

Notice how the verb changes:

• falta with singular uma assinatura
• faltam with plural duas assinaturas

That shows that uma assinatura is the subject.

Because the sentence is referring to one unspecified signature.

• uma assinatura = a signature

This sounds natural and standard here.

If you said just falta assinatura, that can exist in some contexts, especially in more colloquial or reduced speech, but it sounds less neutral and less complete. In a sentence like this, falta uma assinatura is the most standard and natural choice.

No is a contraction of:

• em + o = no

So:

• no formulário = in the form or on the form

Portuguese uses this contraction very often.

Other examples:

• na mesa = em + a = on the table
• nos documentos = em + os = in the documents
• nas páginas = em + as = on the pages

Also, even though English often says on the form, Portuguese commonly says no formulário.

Because formulário here is treated as a specific noun with the definite article o:

• o formulário = the form

When em combines with o, you get:

• no formulário

Portuguese uses definite articles more often than English does. So where English might say on a form or on the form, Portuguese often naturally says no formulário if the form is understood in the context.

Usually, no. Many writers would simply write:

• Esta mensagem é urgente porque falta uma assinatura no formulário.

That is probably the most neutral punctuation.

The comma before porque can appear if the writer wants to mark a pause or give the sentence a slightly more formal rhythm, but in a straightforward sentence like this, it is often omitted.

So:

• without comma = very common, very standard
• with comma = possible, but often more stylistic than necessary

Yes. Both are natural in Brazilian Portuguese.

• falta uma assinatura = slightly more direct, compact
• está faltando uma assinatura = also very common, often a bit more conversational

So these are both fine:

• Esta mensagem é urgente porque falta uma assinatura no formulário.
• Esta mensagem é urgente porque está faltando uma assinatura no formulário.

The meaning is basically the same.

Yes, but the original order is very natural.

Original:

• Esta mensagem é urgente, porque falta uma assinatura no formulário.

You could also say:

• Porque falta uma assinatura no formulário, esta mensagem é urgente.

That version puts the reason first. It is grammatical, but it sounds a bit more deliberate or formal.

The original version is probably the most natural if you first want to state the main point and then explain why.