Voltas — Warranty Extension certificate

I have purchased my window AC Serial Number: 4011237A18KC05490 in the month of May 2019, as per the policy I am supposed to do 2 services from the company in a year to ensure my warranty validity for 5 years,

But now when I book my service, your technician is asking for a warranty Extention certificate, I have written mail, and called several times, but still not get the certificate yet.