Conversion factors for unit on main screen changeable only with BMMH6

What causes this issue?

You want to change the conversion factors of an alternative unit that
appears on one of the main screens (for example, order unit); the unit
itself, however, must remain as it is. For this purpose, the conversion
factors must be given by the separate structure for units of measure
(BMMH6) - in the same way as if you were online.

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

System Response

How to fix this error?

Error message extract from SAP system. Copyright SAP SE.