Message type: E = Error
Message class:  CNV_OM_PRESEL - Messages for LT Object Based Preselection 
Message number: 021
Message text: Start of preselection for workset item: &1 (&2)
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.
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The SAP error message CNV_OM_PRESEL021 typically occurs during the preselection phase of a workset item in the context of data migration or conversion processes, particularly when using the SAP S/4HANA Migration Cockpit or similar tools. This error indicates that there is an issue with the preselection of data for the specified workset item.
Cause: The error can be caused by several factors, including but not limited to: Data Inconsistencies: The data being processed may have inconsistencies or issues that prevent it from being selected correctly.
Configuration Issues: There may be configuration problems in the migration settings or the workset item itself. Missing Authorizations: The user executing the migration may not have the necessary authorizations to access the data or perform the preselection. Technical Issues: There could be underlying technical issues, such as database connectivity problems or issues with the migration tools.
Solution: To resolve the CNV_OM_PRESEL021 error, you can take the following steps: Check Data Consistency: Review the data
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