Processing group & is full (choose a different one)

What causes this issue?

A processing group can contain up to 99 combinations of settlement cost
element / receiver cost center. This number has been exceeded in
processing group &v1&.

How to fix this error?

Select an already existing combination of the settlement cost element
and receiver cost center from processing group &v1&, or select a
different processing group.

Error message extract from SAP system. Copyright SAP SE.