how to change base unit of measure

Hello experts,

How to change base unit of measure? I created new meterial. there are no stocks are available for the material. there are no existing purchase requisitions, purchase orders, or scheduling agreements for the material.

But when I used T-code MM02 to change base unit of measure, it displays unit of measure PC used as alternative unit. I can't change it.

Could anyone please help me to look into this issue?

Thanks a lot.