I see two potential problems. The language code is in ISO 639-1 format while the country/region code is in ISO 3166-1 Alpha 2 format.
Issue 1 (minor):
- ISO 639-1 should be written in lowercase
en
- ISO 3166-1 Alpha 2 should be written in uppercase
GB
- Example:
en-GB
Issue 2 (major):
You seem to be using `rel="alternative" which doesn't exist:
SOURCE
Error: Bad value alternative for attribute rel on element link: The
string alternative is not a registered keyword.
Change all alternative occurrences to alternate
. I also recommend that you use a proper markup checker rather than screaming frog, such as W3C validator. By doing it'll help you narrow down problems with more explanation, since its obvious that Screaming Frog failed you in this instance.