DocuSign JWT: "invalid_grant / issuer_not_found" error after successful consent

I'm trying to set up JWT Grant authentication in the DocuSign sandbox, but I'm constantly running into an invalid_grant / issuer_not_found error, even though I have successfully completed the consent process.

My Configuration

• Environment: Development (account-d.docusign.com)

• Integration Key: 1b490ae4-97d1-4b5b-a4d4-cee9998fdf03

• API Account ID: 73228f49-0509-4526-ac11-20a1025960e3

• User ID (the user who granted consent): 80a3a400-b695-4fbf-9809-7cf4538d9419

Problem Description

My server-side application (a Firebase Cloud Function on Node.js) is using the official docusign-esign library to call requestJWTUserToken. In response, I receive a 400 Bad Request error with the following body:

{
  "error": "invalid_grant",
  "error_description": "issuer_not_found"
}

What I've Already Tried

1. Verified all IDs: I have repeatedly checked the Integration Key, API Account ID, and User ID against the details on the "Apps and Keys" page. They are correct.

2. Verified the endpoint: My code is using account-d.docusign.com, which is correct for the sandbox environment.

3. Granted Consent: I successfully navigated to the consent URL below, logged into my developer account, and clicked "Accept".

   • URL: https://account-d.docusign.com/oauth/auth?response_type=code&scope=signature%20impersonation&client_id=1b490ae4-97d1-4b5b-a4d4-cee9998fdf03&redirect_uri=https://www.docusign.com

4. Configured Redirect URI: In my application's settings, https://www.docusign.com has been added as an allowed Redirect URI.

5. Verified the private key format: The private key is being used in the full PEM format and is read directly from a file on the server to eliminate any formatting issues.

My Question

Why is the system unable to find the user for User ID (80a3a400-b695-4fbf-9809-7cf4538d9419) when using Integration Key (1b490ae4-97d1-4b5b-a4d4-cee9998fdf03) after consent has been successfully granted? Are there any additional steps or potential issues on the DocuSign side that could be causing this error?