For OpenText Core SCA to be able to generate Pull Requests on your behalf, you should do some configuration based on the platform:
GitHub: To enable pull requests, you are required to integrate with GitHub using the GitHub app.
GitLab: if you have set up an integration to GitLab, to enable merge requests, you should provide additional credentials (as explained in the article).
Azure DevOps: if you have set up the Azure DevOps integration, to enable pull requests, you must provide additional credentials (as explained in the article).