You have to also create a username in BI Publisher. The username must be same as your P6 username. This Username in BI Publisher must be also granted access to P6 REPORTS.
For example in P6 if your login name is 'admin' , in BI Publisher the a username 'admin' has to be also added and this user must be granted access to P6 REPORTS.
You may review the below notes on MoS
How To Configure P6 EPPM R8 for Enterprise Reporting With Oracle BI Publisher (Doc ID 1276917.1)
Error: "No Data Is Available Because Your P6 Username Is Not Valid For Connecting To The Reporting Database" When Clicking On The Reports Tab in P6 R8.x Web Access (Doc ID 1278903.1)
Let us know if you have problem even after that.