Your Idp needs to support SLO and would need to advertise that fact in it's metadata. A lot of Idp's do not support SLO, or do it in a way that does not match Tableau's expectation.
Are you able to configure Siteminder for SLO? Also - be careful with what you mean by SLO - many customers don't really want SLO they want Tableau Sign Out but to leave the Idp session alive. That can be tricky to set up.
So similar to how the idp metadata has a singlesignonservice element with the attribute set to post, there should be a similar element for singlesignout or singlelogout? I was wondering if that was the case, but couldn’t find any reference to that in the Tableau Server Online help document in the SAML Requirements section.
I will need to check with our idp admin to see if SiteMinder can be configured for SLO.