How to assign a value for Unit of measure (like PC, KG...) in ABAP program?

Hi,

How to assign a value for Unit of measure (like PC, KG...) in ABAP program?

I want to assign PC in a field to execute the program, like wa-ENTRY_UOM = 'pc'.

But the system returen a message that "Unit of measure is not convertible to stockkeeping unit PC"

Could anyone tell me how to assign the value??

Thanks!