Name,ICO,Max,USD_ROI
Bitcoin,0.0,19535.7,N/A
Ethereum,0.0,1389.18,N/A
Ripple,0.0,3.6491,N/A
Bitcoin Cash,0.0,4091.7,N/A
EOS,0.99,21.4637,2068.05%

BTC_ROI,ETH_ROI
N/A,N/A
N/A,N/A
N/A,N/A
N/A,N/A
N/A,N/A
-260.77%,-130.75%

import pandas as pd

f = open("pt1.txt","w+")
df = pd.read_csv("pt2.txt")
f.write(df["BTC_ROI"])
f.write(df["ETH_ROI"])

TypeError: write() argument must be str, not Series

Name,ICO,Max,USD_ROI,BTC_ROI,ETH_ROI
Bitcoin,0.0,19535.7,N/A,N/A,N/A
Ethereum,0.0,1389.18,N/A,N/A,N/A
Ripple,0.0,3.6491,N/A,N/A,N/A
Bitcoin Cash,0.0,4091.7,N/A,N/A,N/A
EOS,0.99,21.4637,2068.05%,-260.77%,-130.75%

df1 = pd.read_csv('pt1.txt')
df2 = pd.read_csv('pt2.txt')

(pd.concat([df1, df2], axis=1, ignore_index=True)
  .to_csv('pt2.txt', index=False, na_rep='N/A')