Message type: E = Error
Message class:  VK - Pflege und Bearbeitung von Konditionen 
Message number: 656
Message text: Specify the ID for the unit of measure
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 VK656, "Specify the ID for the unit of measure," typically occurs in the context of pricing or condition records in SAP, particularly when dealing with sales and distribution (SD) or materials management (MM) modules. This error indicates that the system requires a specific identifier for the unit of measure (UoM) but it has not been provided.
Cause: Missing Unit of Measure ID: The most common cause of this error is that the unit of measure ID has not been specified in the relevant transaction or configuration. Incorrect Configuration: There may be an issue with the configuration of the material master or pricing conditions where the unit of measure is not properly defined. Data Entry Error: During data entry, the user may have overlooked entering the unit of measure ID.
Solution: Check the Transaction: Ensure that you are entering the unit of measure ID in the appropriate field. If you are creating or
 Get instant SAP help. Sign up for our Free Essentials Plan.
Get instant SAP help. Sign up for our Free Essentials Plan. 
VK655 Class & is not valid
Self-Explanatory Message Since SAP believes that this specific error message is 'self-explanatory,' no more information has been given.The majority of...
VK654 There are no selection options for the specified class
Self-Explanatory Message Since SAP believes that this specific error message is 'self-explanatory,' no more information has been given.The majority of...
VK657 Multiple classification is not allowed
Self-Explanatory Message Since SAP believes that this specific error message is 'self-explanatory,' no more information has been given.The majority of...
VK658 System error during classification
Self-Explanatory Message Since SAP believes that this specific error message is 'self-explanatory,' no more information has been given.The majority of...
Click on this link to search all SAP messages.