When attempting to activate a new partnership and the activation is failing with the following error:
Error: Error Activating Partnership <partnership name>
A SAML 2.0 Auth Scheme (Legacy Federation) contained the same IDPID that the partnership was using, thus why the partnership could not be activated.
Although the AdminUI's server.log did not contain any specific information about the error, the smps.log on the Policy Server indicated a conflict with the SAML auth scheme in question.
Deleting the unused SAML 2.0 auth scheme allowed the partnership to be activated.