Если я правильно понимаю,
df = pd.read_clipboard(sep='|')
df['Transcript'] = df['Conversation_ID'].str.split(':',expand=True)[1] # split by delim.
df['Conversation_ID'] = df['Conversation_ID'].str.split(':',expand=True)[0]
print(df)
Conversation_ID Transcript
0 abcdef None
1 BOT Some text.
2 CHATTER Some text.
3 BOT Some text.
4 BOT Some text.
5 CHATTER Some text.
6 BOT Some text.
7 BOT Some text.
и для ожидаемого результата:
new_df = (pd.crosstab(df['Conversation_ID'].iloc[0],
df['Conversation_ID'].iloc[1:],
values=df['Transcript']
,aggfunc='sum')).rename_axis('')
print(new_df)
Conversation_ID BOT CHATTER
abcdef Some text. Some text. Some text. Some text. S... Some text. Some text.