El único requisito es llevar el justificante del pago y una foto reciente.

Because requisito is a masculine noun in Spanish, so the article and adjective must agree with it:

• el requisito
• único requisito

Even though requirement is not gendered in English, Spanish nouns always have grammatical gender.

Here único means only or sole.

So el único requisito means the only requirement.

It is an adjective, not an adverb. It describes requisito.

The written accent in único shows the stress falls on the first syllable:

• Ú-ni-co

Without the accent, the normal stress rules would make you expect the stress somewhere else. So the accent helps show the correct pronunciation.

After es, Spanish often uses an infinitive to explain what something consists of.

So:

• El único requisito es llevar...
• literally: The only requirement is to bring/carry...

This is a very natural structure in Spanish. English does something similar with is to bring.

This is a very common question.

In Spanish, llevar often means to take or to carry with you, but in many contexts it is also used where English prefers bring.

Here, llevar means something like:

• to bring along
• to have with you
• to take with you when you go

So llevar el justificante del pago y una foto reciente means you need to bring/take along those documents.

Using traer would focus more on bringing something toward the place or person speaking. In administrative instructions, llevar is very common.

Justificante is a very common word in Spain. It means a document that proves something.

In this sentence, el justificante del pago means:

• proof of payment
• payment receipt
• document showing that the payment was made

It is a more formal or administrative word than a basic word like recibo in some contexts.

Del is the contraction of:

• de + el = del

So:

• el justificante del pago
• literally: the proof of the payment

Spanish normally contracts de el into del.

Using el pago makes it sound like a specific payment that both sides understand from the context, for example:

• the application fee
• the registration payment
• the bank payment already mentioned elsewhere

So el justificante del pago sounds like the proof of the payment required for that process.

In Spanish, adjectives usually come after the noun.

So the normal order is:

• una foto reciente = a recent photo

Putting reciente before the noun would sound unusual here. After the noun, it is the neutral, standard way to describe it.

Because the sentence is asking for a photo, not one specific photo already identified.

• una foto reciente = a recent photo

If it said la foto reciente, it would suggest a particular photo already known to both speaker and listener, which is not the idea here.

Because foto is short for fotografía, which is feminine.

So we say:

• una foto
• la foto
• foto reciente

This is one of the useful exceptions to the usual pattern that many -o nouns are masculine.

Yes. Both are correct:

• una foto reciente
• una fotografía reciente

Foto is simply the shorter, more common everyday form.

It is especially common in Spain, particularly in formal or administrative language.

In other Spanish-speaking countries, people might also understand it, but depending on the country they may more naturally use other expressions such as:

• comprobante de pago
• recibo
• constancia de pago

For Spanish from Spain, justificante del pago sounds very natural.

Yes, but it would create a different structure.

• El único requisito es llevar...
  This is concise and formal: The only requirement is to bring...

• El único requisito es que lleves...
  This is also correct, but it is more directly addressed to you: The only requirement is that you bring...

In official instructions, the infinitive version with llevar is very common.

It breaks down like this:

• El único requisito = the only requirement
• es = is
• llevar = to bring / to take along
• el justificante del pago = the proof/receipt of payment
• y = and
• una foto reciente = a recent photo

So the pattern is:

• [noun phrase] + es + [infinitive phrase]

That is a very common structure in Spanish.