Purchase Date Not Validated

It means that Apple has no record of the sale. Either the information wasn't correctly entered by the vendor when the phone was first sold, or it's an invalid serial number for a number of reasons. Either way, you'll need to do as it says, contact Apple Support. Since you aren't the original purchaser and so won't have an original receipt unless the person you bought the phone from can provide one, Apple Support may not be able to anything to help, but that's your only option.

Regards.

I've bought this iPhone XS from another person.

That is likely the issue. When you buy a used phone or a phone for a non-Apple seller, you have not idea of its history.

Yours has likely been serviced by a non-Apple provider who failed reset the serial number to the original. IF it has been serviced by someone else, Apple may elect not to further service it.

There is one history clue on your phone. Do Settings > General > About to get something like this:

Note the FIRST character in the model number—in the example, it is "M." This is how the frist letter translates:

• If the model number starts with M, it was purchased new from Apple.
• If the model number starts with F, it was refurbished by Apple or a carrier.
• If the model number starts with P, it was sold as a personalized device with Apple engraving.
• If the model number starts with N, Apple provided it as a replacement for a device device returned for service.

If the number is MISSING, I would be very concerned about the phone's provenance.

Regardless, I recommend you get get your money back and buy only from Apple or a recognized worldwide carrier.