If you are able to successfully connect to your OBIEE server and also able to connect from your Tableau desktop to Subject Area but couldn't connect to reports, then one of the following might be the reason:
1) DSN not created for reports
Check if you have created a separate DSN for OBIEE reports which is a mandatory step. If not, follow the steps in this article to create a separate DSN for OBIEE reports
2) Reports metadata not preloaded
Check if you have preloaded the metadata for OBIEE reports (after the specific report is created in your OBIEE server) which is also a mandatory step. If not, follow the steps in this article to preload metadata for OBIEE reports
3) Schema not selected in Tableau
In the Select Schema of Tableau, you should search for a specific Report by name, select the report and then go to the Tables and choose the Report. You can refer to this video to proceed with the right steps.
4) No appropriate privileges to access the report
Check if you are able to access any other report from your OBIEE environment. If you can connect to other reports, then there must be some permission issues for the logged in user to access that specific report alone. You can confirm this by directly trying to access that report in your OBIEE server from a browser and contact your admin accordingly.