You need a working certificate to put in the Single Sign-On section of Admin to get your SSO working.
Pull the information contained in the tags: <ds:X509Certificate> . Copy and paste the text into a plain text file (Not a Word doc) then give it an extension of .cer or .cert. You should then see it displayed in file explorer with a certificate icon rather than a txt icon. When you double click to open the certificate, you should see a window open with three tabs (General, Details, and Certification Path). Click the details tab and choose "Copy to File...". Walk through the guided steps choosing "Base-64 Encoded" when given the option and give the file any name you want. The certificate generated from this process can be uploaded into the Marketo admin section for SSO.