There are two reasons you might get this error:
This isn't actually a valid ISSN, and it's a typo or some other error on your end. Solution: double-check to make sure the ISSN is legit, and replace/correct it if needed. Then reupload your file.
The ISSN is good, we just don't have the correct data for it loaded on our end. Solution: get in touch with us and we'll fix it. The easiest way to get in touch is using the contact button at the bottom right of your screen, but you can also shoot us an email at firstname.lastname@example.org.