Conditional Calculation of Pandas Dataframe colums

I have a pandas dataframe which reads

Category  Sales  
A           10
B           20

I wanna do a conditional creation of new column target

And I want my target df to look like

Category  Sales  Target 
A           10    5
B           20   10

I used the below code and it threw an error

if(df['Category']=='A'):
    df['Target']=df['Sales']-5
else:
    df['Target']=df['Sales']-10

Am I doing anything wrong, I appreciate your time and effort to help me.