Except the field tells me I use invalid characters when I input my all numbers phone number. Which is kinda wild because what the fuck else is a phone number supposed to have if not numbers :V
Some sites demand the "+<country code>", rather than whatever
your international prefix is ("00", here, in +44 land, but can't quite remember what +1 land uses). Others might think the "+" is wrong (but you'd mention if that was included), or handily give an all-countries drop-down choice if you need it (which ideally just leaves the rest of the number to be subject to being 'text validated', unless you deliberately get trickey with the HTML source before submitting...)
And, after the international code, you may or may not need the first digit of the area prefix (again, from UK experience, we drop the "area" 0).
Of course, if you're just putting in (say) "01234 567890" as your number, or whatever it is, you may be falling foul of it assuming you are giving it a US "123 456 7890" style number, in which it's not so much as that you have a non-digit, but that you have a digit that it doesn't expect (or even extra ones).
Without checking B&N myself (who should have done enough trading like this to make it plain enough what they need from what you know), can't really tell what they're looking for. It could also be a
space (or lack of it, it used to be a crapshoot whether postcodes of form "AB1 2CD" could be entered in forms as that or "AB12CD", until the validation routines got that little bit more smart about the Area+District/Sector+Unit separation).
Check that you have (by this point) definitely geolocated yourself in <your country>, address-wise, and any modern international-aware webform should perhaps autofill (or make a changable default) and reconfigure its fields to accept what its knowledge is about generally valid phone numbers where you are. (Might be caught out by the odd anomaly, but can't imagine that happens frequently enough if you haven't suddenly had your Subscriber Trunk Dialling codes redone, recently).
Also, and again with no knowledgs of B&N's system, but that message might mean that the verification mail it tried to send to your email address (supplied by you, long before you bothered to try the phone) was not received/acted upon at your end. Check your spam folders, that you gave
the right address, etc.
Thirdly, though you probably only had this possibility at any actual payment stage, if I do any card payments online I get a pop-up from my card-issuer getting me to select the (only!) relevent mobile number registered to itz so that they can send it an authorisation code which I need to enter back into the popup window as a 2nd(ish) factor authentication. Should you have got that, but didn't because of pop-up-blockers, and because you didn't fulfil the request it declined payment, thus the purchase? (The pop-up is essentially cross-site, in that it's not the ecommerce site's business what you do next, just that it must get a "Yes, the card-issuer is happy to continue" response. Different jurisdictions may require/allow totally different levels of cross-confirmation.)
Probably some other possibilities, but all the above give as food for your thoughts. Might or might not inspire a Eureka moment, even if none of the above is on target. And it's about five years since I last bought a book online, and that was neither B&N (which I've never used) nor Amazon (which I forced myself to stop using, more than a decade ago).