Here are two corrected versions (one neutral, one slightly more formal) plus a short, simple explanation of the changes.
Neutral correction:
Thank you for your response. Could you please confirm the name of your client: Siargei Slesarenka? Frankly, we are surprised to hear that he has never worked for AZToys. Otherwise, we understand the situation and will plan the next steps accordingly.
More formal option:
Thank you for your response. Could you please confirm whether your client's name is Siargei Slesarenka? We were surprised to learn that he has never worked for AZToys. In any case, we understand the situation and will plan the next steps accordingly.
What I changed and why (simple step-by-step):
- "Can you" → "Could you": "Could" is slightly more polite in written requests.
- Added a comma after "Frankly": short introductory words like "Frankly" are usually followed by a comma.
- Added "that" after "hear": "hear that" is the full, natural phrase in this sentence.
- "the next steps" is clearer than "next steps" here — adding "the" makes the phrase sound complete.
- Offered a formal variant using "confirm whether your client's name is..." which is a bit clearer and more direct.
- Changed "hear" to "learn" in the formal version — both are correct, but "learn" is slightly more neutral and formal.
Choose the neutral version for a polite everyday tone, or the formal version if you want to sound more professional. If you want, I can make it shorter or more casual.