Müşteri numaranızı yazın; aksi takdirde mükerrer tahakkuku bulamayız.

It breaks down like this:

• müşteri = customer
• numara = number
• numaranız = your number
• numaranızı = your number, as the direct object of the verb

So müşteri numaranızı means your customer number.

A useful comparison:

• müşteri numarası = customer number
• müşteri numaranız = your customer number
• müşteri numaranızı = your customer number, with the accusative ending because it is the thing being written

That final -ı is the accusative case ending, used for a specific/definite direct object.

In this sentence, the speaker is telling you to write a particular number: your customer number. Because it is specific, Turkish marks it.

Compare:

• Bir numara yazın. = Write a number.
• Numaranızı yazın. = Write your number.

The second one is definite, so it takes the accusative.

-nız is the your ending here. It is the 2nd person plural or polite singular possessive suffix.

So:

• numara = number
• numaranız = your number

In instructions, forms, websites, and customer-service language, Turkish often uses this polite form even when speaking to one person.

Yes. Yazın is an imperative form of yazmak = to write.

Here it means write or please write.

More specifically, it is the 2nd person plural / polite singular imperative, so it can mean:

• write, addressing more than one person
• write, addressing one person politely/formally

Compare:

• yaz = write! (informal, singular)
• yazın = write! (polite singular or plural)
• yazınız = write! (even more formal/stiff)

Not in this sentence.

Turkish yazın can indeed also mean in summer, but here the context makes it clearly the verb write.

Since the sentence is giving an instruction, Müşteri numaranızı yazın, the reader understands it as the imperative of yazmak.

It means otherwise or if not.

Literally, it is something like in the opposite case/circumstance, but you should learn it as a fixed phrase meaning otherwise.

It is fairly formal and very common in:

• notices
• official instructions
• contracts
• customer-service messages

Similar expressions are:

• aksi halde = otherwise
• yoksa = otherwise / or else

Mükerrer is a fairly formal word meaning:

• duplicate
• repeated
• entered twice
• occurring more than once by mistake

It is especially common in:

• bureaucracy
• banking
• billing
• legal or administrative language

In everyday speech, people often use simpler wording instead, depending on context.

Tahakkuk is a technical administrative/financial word. Depending on context, it can refer to something like:

• an assessed amount
• an accrual
• a billing entry
• a charge that has been recorded

In this sentence, mükerrer tahakkuk likely refers to a duplicate charge/entry/assessment.

It becomes tahakkuku because it is the definite direct object of bulamayız:

• mükerrer tahakkuk = duplicate accrual/entry
• mükerrer tahakkuku = the duplicate accrual/entry, as the object

Only the noun gets the case ending; the adjective mükerrer stays unchanged.

In customer-service and institutional Turkish, we often means:

• the company
• the office
• the staff
• the system

So bulamayız does not necessarily mean a group of people speaking personally. It often means something like:

• we, as the institution, cannot find it
• our system cannot locate it

This is very common in formal or service-related language.

It comes from bulmak = to find.

Here bulamayız means we cannot find or we will not be able to find.

A helpful comparison:

• buluruz = we find / we will find
• bulabiliriz = we can find
• bulamayız = we cannot find

So this form expresses inability or impossibility.

In this sentence, it has a practical meaning: if you do not provide that information, we won’t be able to locate the duplicate entry.

Yes, it is very normal.

Turkish usually puts the verb near the end of the clause, and objects normally come before the verb.

So both parts are typical:

• Müşteri numaranızı yazın
• aksi takdirde mükerrer tahakkuku bulamayız

The structure is straightforward and natural for an instruction.

The semicolon links two closely related clauses:

• first, the instruction
• then, the consequence if that instruction is not followed

It gives the sentence a formal written tone, which fits official or customer-service language well.

The same idea could also be written in other ways, for example with a full stop or with a different connector, but the semicolon is perfectly natural here.