i have given my macbook pro to a service provider of apple .. they are unable to install OS

That is an issue between you and that service provider. If they are not on Apple's list of Apple Authorized Service Providers, then they should not be touching your Mac. Also, because your Mac is now 12 years old, it has been on Apple's obsolete hardware list for a few years and real Apple Authorized Service Providers may not service it for that reason.

Also, a real Apple Authorized Service Provider would know how to work around re-installing macOS Catalina which is the last supported operating system for that Mac.

You can consider connecting an external drive via a USB connection and installing the OS on it, then make it your startup disk. Keep in mind hat you can only install an OS which will work with your older Mac.

If all else fails...

https://mrmacintosh.com/how-to-fix-the-recovery-server-could-not-be-contacted-error-high-sierra-recovery-is-still-online-but-broken/

High Sierra “The Recovery Server Could No… - Apple Community

Use http:// instead of https://

https://www.youtube.com/watch?v=ouNQS36dIBw&t=95s

The formula for caos: APFS+macOS High+EFI… - Apple Community

How to recover a Deleted Hard Drive durin… - Apple Community