Python2.7：无法使用np.where和np.nan方法创建空值

I'm having difficulty resolving an issue whereby after using np.where to compare 2 row values within a column (position), with the result being assigned to a new column (null value is created if condition is false), i am unable to use fillna method to replace the null values with the values of the newly created columns I'm having difficulty resolving an issue whereb