Do I need to purchase a Power BI license if I purchase a Fabric license?

In most cases, yes. Power BI licences are still required, depending on how users interact with content.

• Report creators need a Power BI Pro or Premium Per User (PPU) licence
• Viewers:
  • With a Fabric F64 or above capacity, users can view reports without a Pro license.
  • With a smaller Fabric capacity (ie F2 to F32), users will require a Power BI Pro or Premium Per User (PPU) licence.

In summary, Fabric provides the platform. Power BI licences control access.