Message type: E = Error
Message class:  REEXPM - Message Texts for Interface to PM 
Message number: 041
Message text: Enter order number
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 REEXPM041, which states "Enter order number," typically occurs in the context of SAP's Project System (PS) or Controlling (CO) modules when a user attempts to execute a transaction that requires an order number, but none has been provided.
Cause: The error is triggered when: A user tries to access a function or transaction that requires an order number (e.g., creating or processing an order) but does not input one. The order number field is left blank or is incorrectly filled. The transaction being executed is not applicable without a specific order number.
Solution: To resolve the REEXPM041 error, you can take the following steps: Input Order Number: Ensure that you enter a valid order number in the required field. This could be a production order, maintenance order, or any other relevant order type depending on the transaction you are executing. Check for Validity: Verify that the order number you are entering is valid and exists
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