select sum(t1.sales) from Store_information t1,Geography t2 
  where t1.store_name = t2.store_name;

------解决方案--------------------
select sum(t1.sales) from Store_information t1 
 where t1.store_name 
 IN （Select Store_name from Geography t2 ，Store_information t1 where t2.store_name = t1.store_name）


------解决方案--------------------
LZ贴下错误吧，表示没看出来错误在哪。
SQL code

select sum(t.operid) from operskill t 
where t.skillid in (
     select skillid from skillinfo t1 where t.skillid =t1.skillid
);

------解决方案--------------------
SQL code

--別用in，效率低
select sum(t1.sales) from Store_information t1  
  where exists (Select 1 from Geography t2 where t2.store_name = t1.store_name);