client assertion format not proper

Question

I'm following this approach : https://learn.microsoft.com/en-us/entra/identity-platform/certificate-credentials#assertion-format to generate client assertion, but client assertion not generated proper. and this is the code example : https://github.com/Azure-Samples/ms-identity-java-daemon/blob/master/msal-client-credential-certificate/src/main/java/ClientCredentialGrant.java and following this to create private key and certificate : https://github.com/Azure-Samples/ms-identity-java-daemon/tree/master/msal-client-credential-certificate