Answering my own question, it was a silly problem that I failed to see
When creating a new aws_organizations_accoun
t it will start out with only the default regions enabled and I was trying to use var.region = "eu-south-1"
which isn’t one of them.
After logging in the AWS Console and enabling the region my code started to work correctly.
I’m now looking into how to enable my non-default region with terraform.