V případě, že debetní karta chybí, zablokujte ji v mobilním bankovnictví.

Yes, in this sentence V případě, že means something like in the event that or simply if.

• v případě = in the case / in the event
• že = that

So literally it is In the case that ...

This is a more formal, administrative style than just using když or jestliže. In banking, official instructions often prefer this kind of wording.

Případě is in the locative singular because it follows the preposition v.

• případ = case, instance
• v případě = in the case

This is a fixed expression, and it is very common in formal Czech.

Because že debetní karta chybí is a subordinate clause.

The structure is:

• main phrase: V případě
• subordinate clause: že debetní karta chybí
• main clause: zablokujte ji v mobilním bankovnictví

Czech normally separates this inserted clause with commas:

V případě, že debetní karta chybí, zablokujte ji v mobilním bankovnictví.

So the commas are standard punctuation here.

Yes, chybí means is missing, is absent, or is not there.

It comes from the verb chybět. In this sentence, it means the debit card cannot be found or is missing.

This is slightly different from:

• ztratila se = it got lost
• byla odcizena = it was stolen

So karta chybí is a broader, practical wording: the card is missing, whatever the reason.

Debetní karta is the standard Czech term for debit card.

In Czech, many adjectives are formed in ways that do not exactly match English expectations. You simply need to learn debetní karta as the normal collocation.

Compare:

• debetní karta = debit card
• kreditní karta = credit card

Zablokujte is the imperative form, meaning block.

More specifically, it is the 2nd person plural imperative, which Czech uses for:

• speaking to more than one person
• or speaking politely/formally to one person

So here it is the equivalent of a polite instruction:

Please block it

This is common in customer-service and official instructions.

Ji means it, referring to debetní karta.

Since karta is a feminine noun, the pronoun must also be feminine:

• karta = she/it (grammatically feminine)
• ji = her/it in the accusative

It is the direct object of zablokujte:

• zablokujte ji = block it

Because ji and jí are different case forms.

Here you need the accusative form, because the card is the direct object of zablokujte:

• ji = her/it in the accusative
• jí is usually dative or instrumental-related usage, not correct here

So:

• Zablokujte ji. = Block it.
  is correct.

It is in the locative because it follows the preposition v meaning in.

• mobilní bankovnictví = mobile banking
• v mobilním bankovnictví = in mobile banking

Here the adjective and noun both show locative singular:

• mobilním = locative singular
• bankovnictví = locative singular form of a neuter noun

Yes, that is the practical meaning.

Literally it says in mobile banking, but in natural English you would often translate it as:

• in the mobile banking app
• through mobile banking
• using mobile banking

So Czech uses v where English may choose a slightly different phrasing.

Czech word order is fairly flexible, but this order is natural and neutral for an instruction.

• zablokujte = the action
• ji = the object
• v mobilním bankovnictví = where/how to do it

So it flows like:

Block it in mobile banking.

Other word orders are possible, but this one sounds standard and clear in a formal instruction.

Zablokujte is from the perfective verb zablokovat.

That matters because the instruction is about completing a single action:

• zablokovat = to block, to carry out the blocking successfully
• blokovat = to be blocking / to block in a more ongoing sense

In instructions like this, Czech often uses the perfective imperative because the goal is a completed result:

Make sure the card gets blocked.

Yes. A less formal version might use když or jestli instead of v případě, že.

For example:

• Když vám chybí debetní karta, zablokujte ji v mobilním bankovnictví.
• Jestli vám chybí debetní karta, zablokujte ji v mobilním bankovnictví.

But the original sentence sounds more official and is exactly the kind of style you would expect from a bank.