Each month Compass updates with information from the GDC register. The creating of performers on Compass is linked to information from the GDC.
If you, a Commissioner (Sustainability and Transformation Partnership / Local Health Body) receives the error message 'GDC number doesn't exist' even though the number can be confirmed on the GDC website, it's because the information hasn't yet updated.
In such cases you can override this message by clicking 'Ok' on the pop-up message itself, and continue creating the performer if you have one of the following access levels: Administrator Plus Primary User, Primary Users or Performer Create and Update only (PCSE Users).