You cannot enter quantity/amount (Several sub-numbers chosen)

What causes this issue?

If the posting relates to several sub-numbers, then you cannot post
a retirment based on quantity or amounts. This is not possible since
the quantity entered cannot be allocated to different assets.

System Response

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

How to fix this error?

Error message extract from SAP system. Copyright SAP SE.