Hi Rajesh,
To what Ashok has mentioned, if you click F1 on the 'Attachment allowed', you will find the below SAP help text. This indicates that GENER/OBJSV Switch must be activated, meaning you must mark it '1' against this switch.
Kindly check this.
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Flag: Attachment Allowed
Use
If you set this indicator, users can add attachments to an appraisal document.
Dependencies
The function is connected to the generic object services.
Note
You can only attach documents if you have activated the switch GENER/OBJSV in table T77S0 System Table.
Courtesy: SAP
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Hope this helps.
Regards
Kumarpal