Registering a RISC receiver endpoint for Google fails with 403 Forbidden "The caller does not have permission"

I'm trying to register an API endpoint to receive RISC security events from Google and have followed their tutorial for doing this. However, I've been struggling to get a successful response from their side and I always get the following unhelpful error:

HTTP 403 Forbidden
Response payload: {
  "error": {
    "code": 403,
    "message": "The caller does not have permission",
    "status": "PERMISSION_DENIED"
  }
}

Now, this error message is nowhere mentioned in their error code reference docs. I've tried researching but there are very few resources on the topic. I tried to apply most of the stackoverflow answers on the topic with no success.

I'm pretty sure I configured everything correctly in the Google Cloud Console. For instance, I have enabled the RISC API for my project:

Additionally, I assigned the right roles and access rights to my service account:

I'm also following their code suggestions to generate the JWT for my account and I think it's being correctly generated because when I change something I get a different error. Here's the code for generating the JWT:

String createJwt() {
    String issuer = "https://accounts.google.com";

    PrivateKey privateKey = myPrivateKey;

    Algorithm algorithm = Algorithm.RSA256(null, (RSAPrivateKey) privateKey);
    Date issuedAt = new Date();
    Date expiresAt = new Date(issuedAt.getTime() + 3600000);

    return JWT.create()
            .withKeyId("MyPrivateKeyId")
            .withIssuer("MyClientEmail")
            .withSubject("MyClientEmail")
            .withAudience("https://risc.googleapis.com/google.identity.risc.v1beta.RiscManagementService")
            .withIssuedAt(issuedAt)
            .withExpiresAt(expiresAt)
            .sign(algorithm);
}

Does anybody know what I'm doing wrong or have any suggestions to fix the problem?