Price phase cannot be deleted as it is the only price phase for the mat.

What causes this issue?

You tried to delete the price phase of a material.
However, only one price phase exists for the selected item.

System Response

The price phase is not deleted.

How to fix this error?

To delete a material completely from the markdown plan, select the item
in rough-cut planning and choose "Delete line".

Procedure for System Administrators

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

Error message extract from SAP system. Copyright SAP SE.