This is almost certainly due to the SHA-1 certificate used by iTunes Store expiring. It is most likely that iTunes and macOS either pinned the old SHA-1 issuing

This is almost certainly due to the SHA-1 certificate used by iTunes Store expiring. It is most likely that iTunes and macOS either pinned the old SHA-1 issuing CA and/or simply don't support SHA-2 at all in this context. (No new SHA-1 certificates may be issued for any reason.) Apple offers free software updates to newer versions of both macOS and iTunes, and has indicated that applying those updates will be necessary to continue accessing iTunes Store. See also, for example, https://news.ycombinator.com/item?id=16467490 https://news.ycombinator.com/item?id=16467490 or https://support.apple.com/en-us/HT208104 https://support.apple.com/en-us/HT208104