Problem

what rule do i need to add to solve this: The username could not be retrieved from the IdP and is required

and what attribute name for the username?

I used the follow attributes:

LDAP Attribute

1. E-Mail-Addresses
2. Given-Name
3. Surname
4. SAM-Account-Name
5. Token-Groups – Unqualified Names

Outgoing Claim Type

1. E-Mail Address
2. Given Name
3. Surname
4. UPN
5. Group

Attribute Mapping

1. https://schemas.xmlsoap.org/ws/2005/05/identity/claims/emailaddress
2. https://schemas.xmlsoap.org/ws/2005/05/identity/claims/givenname
3. https://schemas.xmlsoap.org/ws/2005/05/identity/claims/surname
4. https://schemas.xmlsoap.org/ws/2005/05/identity/claims/upn
5. https://schemas.xmlsoap.org/claims/Group

All claims are working according to a SAML debugger however wordpress throws an Error 500 for the ?saml_acs page even when IDP initiated.

Hi,
I had the same error and when changing wp-options.php: WP_Debug = True, I received:

Fatal error: Uncaught exception 'OneLogin_Saml2_ValidationError' with message 'NameID not found in the assertion of the Response'

My ADFS setup wasn’t exposing Name ID (the username was SAM Account Name). It appears the plugin MUST receive a NAME ID in the SAML Assertion (even if NOT using it for mappings).
I simply added the Name ID in as a claim (using SAM ACCOUNT NAME) and it started working.