Need to select the Cash Flow Assignment. As if you have selected Mandatory in the General Settings.
If you select Optional it not required at the time of reversal.
To access this window from the SAP Business One Main Menu, choose Administration --System Initialization -- General Settings.
Assignment of Cash Flow Line Item: Mandatory
By selecting this field you define a mandatory assignment of a cash flow line item to a cash flow relevant transaction. If a cash flow relevant transaction is not assigned to a cash flow line item, an error message is displayed and the transaction will not be added to the system.
Assignment of Cash Flow Line Item: Optional
By selecting this field you define the optional allocation of a cash flow line item to a cash flow relevant transaction.
You can further check Note No. :1702203 and confirm whether your issue is similar to the one mentioned by you.
The workaround to add the outgoing payment is to change the option to 'Optional' from 'Mandatory' in the Cash Flow tab of the General Settings under the Admininsitration Module.
Check the working in the COPY of the database.
SAP Business One Forum Team