How to add and subtract column in pandas

Question

I am trying to subtract two columns in my data frame and add a new column called delta to represent this

I tried the below code but I get the below error message:

KeyError: 'Tday'

enter image description here here is my data

import pandas as pd

df=pd.read_csv("P:\\Operators\\Matt\\Python\\ResidualLoad1.csv",encoding='latin1')
df['delta'] = df['Tday'] - df['DA']
print(df)