I have found a solution. I have replaced "N" with "-1" by using this code:

df2 = df2[df2['Model Year'] != 'N/']  # Filter out rows where 'Model Year' is 'N/'

for col in cols:
    df2[col] = df2[col].replace('N/', -1)
    le.fit(df2[col])
    df2[col] = le.transform(df2[col]) 
    print(le.classes_)