Si no reps res avui, demà pots trucar a correus o a la secretària.

Because they are two different things:

• si no = if not
• sinó = but rather / except

In your sentence, Si no reps res avui... means If you don’t receive anything today..., so it has to be si no as two words.

Compare:

• Si no reps res avui, demà pots trucar... = If you don’t receive anything today, tomorrow you can call...
• No trucis a correus, sinó a la secretària. = Don’t call the post office, but rather the secretary.

So here si no is correct because it introduces a condition.

Reps is the 2nd person singular present tense of rebre (to receive).

So:

• jo rebo = I receive
• tu reps = you receive
• ell/ella rep = he/she receives

This is an irregular verb, so the form is not something fully predictable from the infinitive unless you learn it.

In the sentence:

• Si no reps res avui... = If you don’t receive anything today...

the subject is an implied tu (you), which Catalan often leaves out because the verb form already shows the person.

This is a very common point of confusion.

res literally corresponds to nothing, but in a negative sentence Catalan often uses it where English uses anything.

So:

• Reps res? = Do you receive anything?
• No reps res. = You don’t receive anything.

Catalan uses this kind of negative concord, where the negative idea is expressed with both no and res together.

So in your sentence:

• Si no reps res avui...

the natural English translation is:

• If you don’t receive anything today...

not If you don’t receive nothing today...

Because Catalan often uses the present tense to talk about future situations, especially in conditional clauses with si.

So this structure is very normal:

• Si plou demà, no sortim. = If it rains tomorrow, we won’t go out.
• Si no reps res avui, demà pots trucar... = If you don’t receive anything today, tomorrow you can call...

English often uses present tense too in the if-clause (If you don’t receive...), but Catalan also comfortably uses the present in the main clause when the future is understood from context.

You could also find future forms in other contexts, but this sentence sounds natural as it is.

pots trucar literally means you can call.

It gives the idea of option / possibility / suggestion:

• demà pots trucar... = tomorrow you can call...

This is softer than:

• truca = call (imperative, a direct instruction)
• trucaràs = you will call (more like a statement about what will happen)

So pots trucar sounds like helpful advice: if nothing arrives today, calling tomorrow is an available next step.

Because in Catalan, trucar normally takes the preposition a before the person or place you are calling.

So:

• trucar a la Maria = to call Maria
• trucar al metge = to call the doctor
• trucar a correus = to call the post office / postal service

This is different from English, where we usually say call someone without a preposition.

So in your sentence both are correct:

• trucar a correus
• trucar a la secretària

Here correus means something like the post office, the postal service, or the mail service, depending on context.

Even though the word looks plural, it is often used in Catalan to refer to postal services in a general institutional sense.

So:

• trucar a correus = to call the post office / postal service

Whether you translate it as post office or postal service depends on the situation. If the context is about a missing letter or package, either could work.

Also, the lowercase correus usually suggests a general/common noun use. If it referred specifically to the Spanish postal company as a proper name, you might see Correus in Catalan contexts.

Yes, they are different words.

• secretària = secretary (the person)
• secretaria = office / department / secretariat

So:

• trucar a la secretària = to call the secretary
• trucar a la secretaria would mean to call the office/department

The accent matters because it helps distinguish the person from the office.

Because Catalan is a pro-drop language, which means subject pronouns are often omitted when the verb already makes the subject clear.

Here:

• reps already tells you the subject is tu
• pots also tells you the subject is tu

So Catalan normally says:

• Si no reps res avui, demà pots trucar...

rather than:

• Si tu no reps res avui, demà tu pots trucar...

You can include tu for emphasis, but it is not necessary.

Yes. Demà is flexible.

Your sentence has:

• Si no reps res avui, demà pots trucar...

This is very natural, but you could also say:

• Si no reps res avui, pots trucar demà a correus o a la secretària.

Both are grammatical. The difference is mainly one of emphasis and rhythm.

Putting demà right after the comma makes the timeline very clear:

• today nothing arrives
• tomorrow you can call

Catalan often repeats the preposition before each item in a list, especially when it sounds clearer or more natural.

So:

• a correus o a la secretària

is perfectly normal.

You may sometimes see a preposition used only once before coordinated elements, but repeating it is very common and often preferred when the two nouns are different in type.

Here it also helps because one is an institution (correus) and the other is a person (la secretària), so repeating a makes the structure very clear.

It is fairly neutral and natural.

The verb forms:

• reps
• pots

are 2nd person singular informal (tu form), so the sentence is addressing one person informally.

If you wanted a formal singular version with vostè, you would change the verbs:

• Si no rep res avui, demà pot trucar a correus o a la secretària.

So the original sentence is not rude or slangy; it is just the normal tu form.

A few useful notes:

• reps: one syllable, roughly like reps
• res: short, roughly ress
• avui: approximately uh-VUI
• demà: approximately deh-MA
• trucar: approximately troo-CAR
• correus: approximately co-RREUS
• secretària: approximately seh-kreh-TA-ri-a

The stressed syllables are especially important:

• demà
• trucar
• secretària

Also, Catalan pronunciation varies somewhat by dialect, so exact sounds can differ, but the stress pattern is very helpful to learn early.