A problem with Pandas dataframe (Need help urgently)

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I have a huge Excel data basis. I want to use Pandas to read and manipulate it

This works

import pandas as pd
xlsx = pd.ExcelFile(r'c:\vinsdeloire\Chinonrougegpsga1.xlsx')
cr1=pd.read_excel(xlsx,sheet_name='Données indiv campagne')# sheet_name= est facultatif
cr2=pd.read_excel(xlsx,sheet_name='Données indiv année civile')
cr3=pd.read_excel(xlsx,sheet_name='pilotage GPS')
cr1

I can have the head simply

cr1.head()

The the column names

for idx,column in enumerate(cr1.columns):
    print(idx,column)

I can use the name of one column

cr1['Rotation stock en nb de mois campagne 2019-2020']

But what I do not understand is that until now a command like this one doesn't work

cr11=cr1.iloc[:-9,29]
cr11

given this error

Location based indexing can only have [integer, integer slice (START point is INCLUDED, END point is EXCLUDED), listlike of integers, boolean array] types

I make certainly a mistake and I cannot give more than my code but I do not see my error. And unfrotunately its very urgent. I will remove this later.

Thanks by advance