Unable to delete AUC in the settlement rule in CJ20n

HI SAP Gurus,

We have a wbs element <operating expense cost collector-Investment>  the receiver mapped in CJ20n is AUC it is supposed to be cost center.

I already mapped cost center and But i am not able to remove AUC as receiver as it was greyed-out.  It says "Rule cannot be deleted".

I tried to reverse settlement posted to WBS but still the same.  

Appreciate if you could advise me on how can i remove the AUC as receiver.  We need cost center as receiver.

Thank you!

Eve