If you downgrade from an Enterprise account to Starter or Pro level, and you have SSO enabled, then this downgrade will disable that feature. You will no longer be able to login via SSO.
The solution is to go to the DivvyHQ login screen and initiate a password reset by clicking the Forgot your password? link. Simply input your email address and click the orange Reset My Password button. This will send you an email with instructions on how to reset your password and enable you to log in.