The shopkeeper lied to you.
All you had to do was turn on the laptop at the store. If Windows 10 does not show as activated there at the store then the laptop does not have a genuine Windows 10 key.
It is that simple!
At this point you have to pay for a Windows 10 key from Microsoft.
This forum is for user-to-user support for U S A.
This forum is not for official business contact Dell anyway.