Combination &/& can no longer be taken into account

What causes this issue?

Within activity type planning, only 330 combinations of cost centers/
activity types can be planned simultaneuosly.
This number of combinations is currently available for use. Therefore,
additional combinations are not allowed.

System Response

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

How to fix this error?

Error message extract from SAP system. Copyright SAP SE.