Why can't I use SELECT ... FOR UPDATE with aggregate functions?

The syntax select . . . for update locks records in a table to prepare for an update. When you do an aggregation, the result set no longer refers to the original rows.

In other words, there are no records in the database to update. There is just a temporary result set.

You might try something like:

<<LOCAL>>
declare
  material_id materials.material_id%Type;
  cost        materials.cost%Type;
  total_cost  materials.cost%Type;
begin
  select material_id,
         cost,
         sum(cost) over () total_cost
  into   local.material_id,
         local.cost,
         local.total_cost 
  from   materials
  where  material_id between 1 and 3
  for update of cost;

  ...

end local;

The first row gives you the total cost, but it selects all the rows and in theory they could be locked.

I don't know if this is allowed, mind you -- be interesting to hear whether it is.