Cannot evaluate fraction &1

What causes this issue?

Fraction &1 of recycling partner &V2& has an index that contains
or consists of non-numeric characters.

System Response

The system can not correctly assign the index to a fraction specified
by the recycling partner.

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?

Maintain the fractions for recycling partner &V2&, so that their
index only contains numeric characters.

Error message extract from SAP system. Copyright SAP SE.