Ваш css-селектор также неверен и дал мне ошибку.
from bs4 import BeautifulSoup
import requests
page = requests.get('https://finance.yahoo.com/quote/BDSI/history?period1=1517033117&period2=1548569117&interval=1d&filter=history&frequency=1d')
soup = BeautifulSoup(page.text, 'html.parser')
rows = soup.find('table',class_="W(100%) M(0)").find_all('tr')
numbers=[]
for row in rows[1:20]:
col = row.find_all("td")
print(col[6].text)
number = col[6].text.replace(',', '')
number = int(number)
numbers.append(number)
avg20vol =sum(numbers)/len(numbers)
print("Average: ",avg20vol)
Выход
650,100
370,500
374,700
500,700
452,500
1,401,800
2,071,200
1,005,800
441,500
757,000
901,200
563,400
1,457,000
637,100
692,700
725,000
709,000
1,155,500
496,400
Average: 808584.2105263158