I want to append an csv file using python pandas to add an array as new column

You are trying to assign a dictionary here, whereas your row requires a list of values. You should try this:

import pandas as pd
list1 = {'Names':[1,2,3,4,5]}

df = pd.DataFrame(list1)
df_csv = pd.read_csv('try.csv')
df_csv['Names'] = list1['Names']
df_csv.to_csv('try.csv', index=False, mode= 'a')

Also, it is not clear why would you need an additional df DataFrame for this?

You can try using concat. Suppose your df_csv has more rows as below then, you can create new dataframe with list1 and concatenate as new column:

import pandas as pd
list1 = {'Names':[1,2,3,4,5]}

# creating dataframe with initial value instead of pd.read_csv
df_csv = pd.DataFrame({'col_1': [100,200,300,400,500,600,700]})
print(df_csv)

Result:

   col_1
0    100
1    200
2    300
3    400
4    500
5    600
6    700

Now, concatenate to df_csv with axis=1 by creating new dataframe with list1:

list1_df = pd.DataFrame(list1)
# concatenate df_csv and dataframe with list1_df
df_csv = pd.concat([df_csv,list1_df], axis=1)
print(df_csv)

Result:

   col_1  Names
0    100    1.0
1    200    2.0
2    300    3.0
3    400    4.0
4    500    5.0
5    600    NaN
6    700    NaN