Quantity in order price unit is set automatically

What causes this issue?

An <GL:purchase_order_price_unit>order price unit</> that differs from
the <GL:order_unit>order unit</> (unit of entry) has been defined in
the purchase order. The goods receipt must be entered in both units of
measure.
The system has calculated the order price quantity from the conversion
factor of the purchase order and the GR quantity entered (in the unit
of entry) and automatically placed it in the field "Quantity in OPUn".
With each change in the quantity in unit of entry, the order price
quantity is automatically adjusted, unless it was changed manually.

How to fix this error?

Check whether the proposed order price quantity agrees with the actual
order price quantity. If not, change the quantity in the field
"Quantity in OPUn".
When you change the order price quantity, the system checks whether the
variance in the order price quantity is permissible.

Error message extract from SAP system. Copyright SAP SE.