Currently, we have neither blocked nor written any support for SSO authentication so it works flawlessly for some providers and not so much for others. This is on our roadmap as we recognize a lot of our users use SSO authentication and we are planning to write support for it.
We recommend that you put your SSO provider's name specifically in our feedback survey so we know how to prioritize SSO support.
SSO is different for every organization. In the meantime, if you are able to, please try disabling your SSO to see if you are able to log in to your account in Shift without it.