No formula is maintained for key &1

What causes this issue?

The key (&v1&), which you have specified for calculating the
variable-size item from the size data, does not have any calculation
formulas specified for it.

System Response

The size of the variable-size item cannot be calculated without a
formula.

How to fix this error?

In iPPE Customizing, selecty <DS:TRAN.S_KA5_12000591>Maintaining
Variable-Size Item Formulas</> and maintain a formula for the specified
key &v1&.

Procedure for System Administrators

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

Error message extract from SAP system. Copyright SAP SE.