Hi Aditya,
It should be possible to remove the attribute from Plng Area in SOP as well, even when its used across planning levels. But am just trying to see if there is any other root cause.
Check if this attribute is 'Root' attribute in any of the planning levels. Removing this attribute, then make the planning level change the planning base of that level, and directly impacting the key figures, which are built on that planning level.
Regards, Krishna