Different data in billing document &1 for contract &2

What causes this issue?

At least one of the following conditions for
<DS:GLOS.2A7E8A9C5C28D311B60F0060B03C2BFF>payment plans</> is not
fulfilled in billing document &V1&:
A definite allocation date could not be determined.
The allocation date in at least one of the billing line items is
different than in the other lines of the document.
Only for payment plans of
<DS:GLOS.AC4825FB7773D411A4950060B03C3B0E>Type</>
<DS:GLOS.EB4CB0AB7473D411A4950060B03C3B0E>BBP procedure</>:
The <DS:GLOS.3526BDE1AFAB52B9E10000009B38F974>portions</> in the
concerned billing documents must match.
A different currency appears in at least one line.

System Response

Since determination of a definite allocation month is required for
determining the relevant month for the payment plan, payment plan
preparation cannot be executed for this contract.
There is no guarantee that all contracts are billed for the customer on
the same day if the portions do not match. There is also no guarantee
that the same payment amount will be determined for all billing dates.
If the currency is not the same, then the bill amount cannot be divided
sensibly.
Invoicing of this billing document and the entire invoicing group has
been terminated.

How to fix this error?

If the contract is to remain allocated to a payment plan, then make
sure that the allocation date, the portion, and the currency remain
clearly identifiable. Otherwise, the payment plan has to be terminated
for this contract, and the information for terminating the payment plan
also has to be clearly defined.

Error message extract from SAP system. Copyright SAP SE.