External sharing is disabled automatically when Axio360 instances are provisioned as a security measure.
Only designated Admin Users can enable "external sharing" via the admin settings.
To find that setting, go to the orange tool icon (refresh if you're not seeing it yet) on the left sidebar:
Click on the Company tile:
Scroll down to the Security settings, and use the first checkbox to select your preference for allowing external sharing:
One note: External sharing of assessments is allowed when the above box is checked and effects sharing capability while "turned on." In other words, once you have shared externally, you could turn that capability back off and not effect previous sharing. It would only effect attempts to share moving forward.