Excess when converting to unit of measure in routing header

What causes this issue?

In the reporting point backflush, the entered unit of measure for the
backflush quantity is converted to the routing header unit of measure.

System Response

In this process, the conversion result exceeded the maximum value
allowed.

How to fix this error?

Carry out the backflush using a smaller quantity.

Error message extract from SAP system. Copyright SAP SE.