Hello
If you have already set MRP type ND, this material won't be planned by MRP anymore, therefore, you must manually delete the purchase requisitions (see thread Unable to delete the Purchase Requisition line item).
If you want to have the purchase requisition deleted by MRP, make sure that there in no requirement, set MRP type PD and run MRP. After that, old purchase requisitions will be deleted and you can set MRP ND again.
BR
Caetano