Error "Incoming SAML message has no valid value for username attribute" During Authentication

Ensure that the IdP is sending a valid attribute that matches the username in Tableau Server. If the attribute matching the Tableau Server username is named something other than username, it will be necessary to configure Tableau Server for the correct attribute. For example, in the following SAML response, the attribute for oid:user and not username:

</saml2:Attribute><saml2:Attribute FriendlyName="glid" Name="oid:user" NameFormat="urn:oasis:names:tc:SAML:2.0:attrname-format:uri"> 
<saml2:AttributeValue xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:type="xs:string">testuser</saml2:AttributeValue></saml2:Attribute>

To update Tableau Server to use a new value to authenticate users:
Tableau Sever on Linux and Tableau Server on Windows 2018.1 and newer:

on the computer running Tableau Server, open the command prompt as an administrator.
Run the following commands in order: 
TSM stop
TSM configuration set -k wgserver.saml.idpattribute.username -v <value>
TSM pending-changes
TSM start