This article shows possible steps to figure out the reason why the configuration of “Advanced Portal Reports” fails to save.
Instructions
This error is most probably caused by the fact that you’re not an admin in the project you are trying to configure. Check whether you actually have admin permissions for your user in the project you are using.
You can also do that check using the following REST call:
<your_jira_instance>/rest/api/2/mypermissions?permissions=ADMINISTER_PROJECTS&projectKey=<projectKey>
Set admin rights for your user for the project and try again.
Another possible problem might be that your browser session has expired - try reloading the page (and re-authenticate if needed).
If the problem still occurs, check your browser log for more details - How to export the browser log messages