The error "Access to the resource is forbidden" arises due to the wrong credentials used during the authentication. Follow the steps below to troubleshoot it.
Click the File tab at the top left of the Power BI Desktop.
Click Options and settings on the menu.
Select Data Source Settings.
Select the data source the “Access to the resource is forbidden” error arises for.
Select the Clear Permissions option on the drop-down menu.
Confirm clearing permissions by clicking Delete
Press the Close button.
Thereafter, you can re-enter the credentials when trying to connect the required data source through Basic authentication.
To copy credentials easily use the 'Copy' icon next to the credentials in the Settings tab.
