The shop had not properly prepared the Macbook Pro 20 for resale.
The Macbook Pro 20 is still linked to the previous owner.
Each Apple device is linked/associated with an Apple ID of the person who did the setup.
Activation Lock prevents anyone who is not the owner from using this Mac.
To unlock the Mac, you need the Apple ID and password of the previous owner, that were used during set up.
So you have to contact the previous owner.
The previous owner has to read -> What to do before you sell, give away or trade in your Mac
What to do before you sell, give away or trade in your Mac – Apple Support (UK)
Then the previous owner has to remove the MacBook from his account, see this Apple article:
Check your Apple ID device list to see where you're signed in
Check your Apple ID device list to find where you're signed in - Apple Support
Then you can sign in with your own Apple ID.
See the following Apple articles:
Activation Lock for iPhone, iPad, and iPod touch
Activation Lock for iPhone, iPad, and iPod touch - Apple Support
How to remove Activation Lock -> How to remove Activation Lock – Apple Support (UK)
Read this before you buy a 2nd hand device.
You can also contact Apple support for help, -> Get Support (apple.com)
You have to show proof of ownership of your product, provide all documentation showing your purchase of the device.
If you do not have additional proof of ownership, contact the reseller. They may be able to provide the original purchase document.
Once Apple receive your proof of ownership, Apple will further review your request and provide an update.
See also -> Keep your receipt after you buy an Apple device
Keep your receipt after you buy an Apple device – Apple Support (UK)
It is 'impossible' for you to contact the previous owner.
Ask the shop solve the problem for you, or ask your money back if possible.