Hi Suresh,
If the key-figure lies in SNP production horizon or Stock transfer horizon you will not be able to delete the key-figure values through planning book. However you will be able to delete the same using product view (/sapapo/rrp3) or thorugh transaction /sapapo/rlcdel or through report /SAPAPO/DELETE_PP_ORDER .
Just check if this is the case.
Regards,
Mukesh Pandey