Hi @vvucic,
ISSNs need to pass a regular expression, and they also have a built-in checksum. That appears to be a valid ISSN, so maybe it’s the dash – are you copy-pasting it? If so, try re-typing the dash to ensure that it’s not an em-dash or some other character that looks like a dash but isn’t the identical character that’s expected.
Regards,
Alec Smecher
Public Knowledge Project Team