In order to interact with Xray using its REST API is necessary to create an API Key. An API key may be created for each Jira user. Each API key contains a Client Id and a Client Secret which can be used during the authentication phase.
If no user appears, please verify the following:
1. Start writing the name of a user on the Create API Key screen:
2. Verify if the user that is trying to create the API key has Browse users and groups global permission.