Message type: E = Error
Message class:  FOT_B2A - Messages Administration Report Electronic Data Transmission 
Message number: 813
Message text: Enter the document date
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The SAP error message FOT_B2A813 typically indicates that a document date is required but has not been provided in the context of a business transaction, such as during the processing of a financial document or a business transaction in the SAP system.
Cause: Missing Document Date: The most common cause of this error is that the user has not entered a document date in the relevant field when trying to create or process a document.
Configuration Issues: There may be configuration settings in the SAP system that require a document date for certain transactions, and if these settings are not met, the error can occur. User Input Error: The user may have overlooked the document date field or entered it incorrectly.
Solution: Enter Document Date: The immediate solution is to ensure that you enter a valid document date in the required field. This date typically represents the date on which the document is created or the
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