To access Report Builder, sign in to ArcGIS Online and click on Report Builder for ArcGIS in the app launcher available in the top menu.

ArcGIS Online App Launcher

For existing Report Builder outputs, you can also use the Configure App button within ArcGIS Online to open the edit version of the report.

Alternatively, you can browse directly to and sign in using your ArcGIS Online named user.

Report Builder sign in

If you are unable to sign in, it may be that the administrator of your ArcGIS Online organization has not assigned a Report Builder license to your user.