Here is what you need to do to resolve this:
First, navigate to Org, then Organization, then People, and open the Org Chart view.
Once there, click the Share Chart icon located in the top-right corner of the page.
Revoke the existing link that is currently active.
Generate a brand new link by selecting your desired validity period for how long you want the link to remain active.
Copy the newly generated link and share it with your vendor.
This should resolve the issue entirely. The new link will be fully accessible to external parties outside your organization, and the error notification you received was related to the old link being expired or invalid.
Please also keep in mind that external org chart sharing is an admin-only feature, so make sure the action is being performed by a user with admin permissions.