En el metro hay tan poca cobertura que no he podido reenviar el documento.

En el metro means in the metro / on the subway. Spanish often puts a place expression first to set the scene.

• En el metro = in the metro, on the subway
• El metro by itself = the metro system / the subway

So En el metro hay... means In the metro, there is... or more naturally, On the subway, there is...

Hay means there is or there are.

It comes from the verb haber and is used to talk about existence:

• Hay cobertura. = There is coverage / signal.
• Hay problemas. = There are problems.

In this sentence:

• En el metro hay tan poca cobertura...
  = In the metro there is so little signal...

Here cobertura means mobile signal / reception / network coverage, not literal physical coverage.

So poca cobertura means:

• weak signal
• poor reception
• little network coverage

In Spain, cobertura is very commonly used for phone signal.

Both are possible, but they do different things.

• muy poca cobertura = very little signal
• tan poca cobertura que... = so little signal that...

The structure tan + adjective/adverb/noun phrase + que is used to express a consequence:

• Hay tan poca cobertura que no he podido reenviar el documento.
  = There is so little signal that I haven’t been able to resend the document.

So tan is used because the sentence continues with que and gives the result.

Because cobertura is a feminine singular noun.

So the adjective/quantifier has to agree:

• poca cobertura
• mucha cobertura

Compare:

• poco tiempo = little time
• poca cobertura = little signal

It means so ... that ...

Pattern:

• tan + adjective/adverb + que
• tanto/a/os/as + noun + que

In your sentence, it is effectively:

• tan poca cobertura que...

This expresses a result:

• so little coverage that...

More examples:

• Hace tanto calor que no puedo dormir. = It’s so hot that I can’t sleep.
• Habla tan rápido que no entiendo nada. = He speaks so fast that I understand nothing.

He podido is the present perfect: I have been able to / I haven't been able to.

• no he podido = I haven’t been able to
• no pude = I couldn’t

In Spain, the present perfect is very common when talking about something connected to the present or recent time. This sentence suggests a recent situation with present relevance: the person still hasn’t managed to resend it.

So:

• No he podido reenviar el documento sounds like I haven’t been able to resend the document
• No pude reenviar el documento sounds more like I couldn’t resend the document in a completed past situation

It is the present perfect of poder.

Structure:

• haber in the present + past participle

Here:

• he = I have
• podido = been able

So:

• he podido = I have been able
• no he podido = I haven’t been able

More examples:

• He hablado = I have spoken
• He visto = I have seen
• He podido entrar = I have been able to get in

In Spanish, no goes before the conjugated verb to make the sentence negative.

So:

• He podido reenviar el documento. = I have been able to resend the document.
• No he podido reenviar el documento. = I haven’t been able to resend the document.

This is standard Spanish word order for negation.

Reenviar means to resend or to forward again, depending on context.

It is made of:

• re- = again
• enviar = to send

In this sentence, reenviar el documento most naturally means to resend the document.

It could refer to:

• sending the file again because it failed the first time
• forwarding it again to someone

Both can work.

• reenviar = to resend / forward again
• volver a enviar = to send again

Reenviar is shorter and very common in digital or office contexts, especially with emails, messages, and documents.

Compare:

• No he podido reenviar el documento.
• No he podido volver a enviar el documento.

Both are correct, but reenviar sounds more compact and natural here.

Yes, Spanish could leave it out if the context is already clear.

• No he podido reenviar el documento. = I haven’t been able to resend the document.
• No lo he podido reenviar. = I haven’t been able to resend it.
• No he podido reenviarlo. = I haven’t been able to resend it.

The full noun el documento is used here for clarity.

That would change the meaning.

• No he podido reenviar el documento.
  = I haven’t been able to resend the document.
• No puedo reenviar el documento.
  = I can’t resend the document.

The first talks about what has happened up to now.
The second talks about a current inability right now.

Both are possible in real life, but they mean slightly different things.

It follows this pattern:

En + place + hay + tan + noun phrase + que + result

Breakdown:

• En el metro = in the metro
• hay = there is
• tan poca cobertura = so little signal
• que = that
• no he podido reenviar el documento = I haven’t been able to resend the document

So the logic is:

In the metro there is so little signal that I haven’t been able to resend the document.

Yes, it sounds natural in Spain.

A speaker in Spain would commonly say:

• cobertura for signal/reception
• metro for subway/underground in cities that have one
• he podido for a recent past situation with present relevance

So the sentence is standard and natural European Spanish.