Formal error: Installment plan allows only one partner and one contr.acct

What causes this issue?

Program error during creation of installment plans. You must ensure
that the same business partner and contract account is entered in all
installment plan items. This condition has not been fulfilled in this
particular document.

System Response

Error message. Processing is terminated.

How to fix this error?

INCLUDE NA_SYSADMIN1 OBJECT DOKU ID TX

Procedure for System Administrators

INCLUDE FICA_NA_OSS OBJECT DOKU ID TX

Error message extract from SAP system. Copyright SAP SE.