Message type: E = Error
Message class:  FKKEXC - Message Class for External Payments 
Message number: 006
Message text: Payment on account indicator &2 for payment &1 cannot be evaluated
Since SAP believes that this specific error message is 'self-explanatory,' no more information has been given.The majority of messages in the SAP system have a message text, however this is frequently insufficient to comprehend or resolve the problem.
To make things easier, more detailed information is frequently added to describe the issue, how to fix it, and the necessary steps or configuration modifications.
Unfortunately, there isn't any extra information in this error notice.
First, use our AnswerBot below to get a possible cause and solution (requires a premium subscription).
Also, review the in-depth Common Questions & Answers listed below; you could discover a solution there or be able to connect with others who have faced similar challenges.
You can also try searching the SAP support portal (support.sap.com) but you need a special user ID to access it. It is possible that an SAP support note exists that provides additional details about the mistake or even steps for fixing it.
The SAP error message FKKEXC006 indicates that there is an issue with the payment on account indicator for a specific payment. This error typically arises in the context of contract accounts receivable and payable (FI-CA) when the system is unable to evaluate the payment on account due to certain conditions not being met.
Cause: Invalid Payment on Account Indicator: The payment on account indicator specified in the payment document may not be valid or may not correspond to the expected configuration in the system. Missing Configuration: The configuration for handling payments on account may not be set up correctly in the system. Inconsistent Data: There may be inconsistencies in the data related to the payment or the account, such as missing or incorrect entries in the relevant tables. Payment Document Issues: The payment document itself may have issues, such as being incomplete or incorrectly processed.
Solution: Check Payment on Account Indicator: Verify that the payment on account
 Get instant SAP help. Sign up for our Free Essentials Plan.
Get instant SAP help. Sign up for our Free Essentials Plan. 
FKKEXC005 Payment ID can have a maximum of 16 characters; &1 is larger
Self-Explanatory Message Since SAP believes that this specific error message is 'self-explanatory,' no more information has been given.The majority of...
FKKEXC004 Unable to interpret payment amount &1 &2 of payment &3
Self-Explanatory Message Since SAP believes that this specific error message is 'self-explanatory,' no more information has been given.The majority of...
FKKEXC007 XI system error: &1
What causes this issue?  A system error occurred during the processing of an XI message.System Response  The processing of the XI message was terminat...
FKKEXC008 XI application error: &1
What causes this issue?  An application error occurred during the processing of an XI message.How to fix this error?  For further information about th...
Click on this link to search all SAP messages.