serial number isn’t valid

Hello,

I bought an iPhone 8 Plus in USA.While travelling to India I broke the screen so I went to so called apple service provider.He checked the warranty online and told me that he can replace my phone and asked me to logged out of my apple id and reset my phone. I did that and gave him a phone.He keeps giving me fake promises since last month.

I tried to enter my serial number on https://checkcoverage.apple.com and now it says that

"We’re sorry, but this serial number isn’t valid. Please check your information and try again."

My question is that what happened to serial number or why I am getting this message,Is it possible to change the serial number or delete from apple website ? What are the possibilities ?

Thanks