Hello Reenu,
You may enter OK command "FMOD" and press enter in FTR_EDIT when you display or change the transaction, it will tell you if there is field modification assigned for the product type/trasaction type / activity.
Goto customizing Treasury and Risk Management > Transaction Manager > General Settings > Define Field Selection to check more details.
Another posibility is to check if you have performed "Edit flow" for relevant fows, manual change will block the flow. You may "Undo: edit flow" first, then you can change the term end date.
Or if variable interet used, please check the interest adjustment.
Best regards,
Jasmine