How to configure SAML SSO with Ping?

I am configuring SSO with Ping. Here is the configuration:

_saml_config = {
    'sp': {
        'entityId': 'indico-saml',
        'x509cert': '...',
        'privateKey': '...',
    },
    'idp': {
        'entityId': 'link:ping.domain',
        'singleSignOnService': {
            'url': 'link:/idp/SSO.saml2',
            'binding': 'urn:oasis:names:tc:SAML:2.0:bindings:HTTP-Redirect'
        },
        'singleLogoutService': {
            'url': 'link/idp/ARS.ssaml2',
            'binding': 'urn:oasis:names:tc:SAML:2.0:bindings:HTTP-Redirect'
        },
        'x509cert': '...',
    },
    'security': {
        'nameIdEncrypted': False,
        'authnRequestsSigned': True,
        'logoutRequestSigned': True,
        'logoutResponseSigned': True,
        'signMetadata': True,
        'wantMessagesSigned': True,
        'wantAssertionsSigned': True,
        'wantNameId' : True,
        'wantNameIdEncrypted': False,
        'wantAssertionsEncrypted': False,
        'allowSingleLabelDomains': False,
        'signatureAlgorithm': 'link:www.w3.org/2001/04/xmldsig-more#rsa-sha256',
        'digestAlgorithm': 'link:www.w3.org/2001/04/xmlenc#sha256'
    }
}
AUTH_PROVIDERS = {
    'saml': {
        'type': 'saml',
        'title': 'SAML SSO',
        'saml_config': _saml_config,
        # If your IdP is using ADFS you may need to uncomment this. For details, see
        # https://github.com/onelogin/python-saml/pull/144
        # 'lowercase_urlencoding': True
    }
}
IDENTITY_PROVIDERS = {
    'saml': {
        'type': 'saml',
        'title': 'SSO',
        'mapping': {
            'first_name': 'givenName',
            'last_name': 'sn',
            'email': 'mail',
            'affiliation': 'company',
        },
        'trusted_email': True,
        'identifier_field': 'Username',
    }
}

The attributes in Ping are

I got error:

...
File "/opt/indico/.venv/lib/python3.9/site-packages/indico/core/auth.py", line 116, in handle_auth_error
fn('Authentication via %s failed: %s (%r)', exc.provider.name if exc.provider else None, exc_str,
...
indico-uwsgi[272130]: Message: 'Authentication via %s failed: %s (%r)'
indico-uwsgi[272130]: Arguments: (None, 'SAML login failed', None)

Could you please advise on this issue? Thanks!