@KCUK, Welcome to the HP Support Community!
If the printer is already enrolled in Instant Ink, the previous owner should delete the printer from his/her account for you to register under your account.
We may also try to restart the web services and check if that helps.
Click here to know how to turn off web services. Refer to this document to know how to turn it on.
Refer to this document - Enrolling in HP Instant Ink
Hope this helps! Keep me posted.
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Have a great day!
KUMAR0307
I am an HP Employee