That message on a used PC would indicate the seller cleaned out the hard drive and there is nothing installed. Assuming there is actually a drive still there, you will need to install Windows or a OS of your choice.
If the PC had Windows 10 installed before, you can install it again with the install media.
If you need the install media for Windows 10, you can make it from this Microsoft download
https://www.microsoft.com/en-us/software-download/windows10
With a working PC, scroll down to the "Using the tool to create installation media"
and then follow the directions. You can make either the DVD or USB.
When the install asks for the product key, click the I don't have a product key option to continue,
then the install will continue It will be activated once it connects to the internet, as it uses a digital key.
I'm not an HP employee.
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