Unable to modify " Receiving Inventory Account " and the " Clearing Account "

Hello to all,

 

We meet a problem of parameter setting in the PO module. During the parameter setting of a new operating unit, we wish to modify the " Receiving Inventory Account " and the " Clearing Account " in the screen "Receiving options" but these fields are not modifiable (cf. screen below). 

 

 

I clarify that no modification was made in this screen.

I think that it’s due to a problem of parameter setting but I do not know where.

Somebody knows why these fields are protected in modification and how to raise these limitations?

Thank you in advance for your help.