Message type: E = Error
Message class:  HRFPM_FTE - New Position Management 
Message number: 728
Message text: Assignment key date must be between comparison key date and key date
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The SAP error message HRFPM_FTE728 indicates that there is an issue with the assignment key date in relation to the comparison key date and the key date in the context of the SAP Human Resources (HR) module, specifically in the area of Personnel Management or Flexible Personnel Management (FPM).
Cause: This error typically occurs when the assignment key date (the date you are trying to use for an assignment) does not fall within the range defined by the comparison key date and the key date. The comparison key date is often used to evaluate the validity of certain data, and if the assignment key date is outside of this range, the system will throw this error.
Solution: To resolve this error, you can take the following steps: Check Key Dates: Verify the key date and comparison key date in your transaction. Ensure that the assignment key date you are trying to use is indeed between
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