IDX10500: Signature validation failed. Unable to resolve SecurityKeyIdentifier
From the error, I think you need to add an x509 Security key or credentials, something like this:
var credentials = new X509CertificateCredentials(
Certificate.Get(),
new SecurityKeyIdentifier(
new NamedKeySecurityKeyIdentifierClause(
"kid",
"6B7ACC520305BFDB4F7252DAEB2177CC091FAAE1")));
eg this part:
new SecurityKeyIdentifier(
new NamedKeySecurityKeyIdentifierClause(
"kid",
"6B7ACC520305BFDB4F7252DAEB2177CC091FAAE1")
Also, make sure your certificate is installed in your root store.