certificate working with clientCredentialType="Windows" but not "UserName"

Hello - I have a serviceCertificate configured for my WCF service. I'm able to successfully update the service reference from my console app with the following configuration:

<wsHttpBinding>
<binding>
<security mode="Message">
<message clientCredentialType="Windows" />
</security>
</binding>
</wsHttpBinding>

However, the system returns an error when I try to update the service reference from my console app with the following configuration:

<wsHttpBinding>
<binding>
<security mode="Message">
<message clientCredentialType="UserName" />
</security>
</binding>
</wsHttpBinding>

The error returned is:  "It is likely that the certificate 'CN=RootCATest' may not have a private key that is capable of key exchange or the process may not have access rights for the private key."  Any idea why I get this error for clientCredentialType "UserName" but not "Windows"?  Does clientCredentialType "Windows" not use the certificate?