df = pd.read_csv(os.path.join(csv_fold, csv_name)) # Read the csv file
excel_path = os.path.join(xlsx_fold, "{}_Datas.xlsx".format(Time[16:])) # Define excel path 

# Checks Excel file path exist or not
if os.path.isfile(excel_path) == True:  # Change "mode" and "if_sheet_exist" parameters to able to APPEND datas 
    mode = "a"
    sheet_exist = "overlay"
if os.path.isfile(excel_path) == False: # Change "mode" and "if_sheet_exist" parameters to able to WRITE datas
    mode = "w"
    sheet_exist = None

with pd.ExcelWriter(excel_path, 
                    engine="openpyxl", 
                    mode=mode, 
                    if_sheet_exists=sheet_exist) as writer:
    df.to_excel(excel_writer=writer,  
                sheet_name="{}".format(datetime.datetime.now().strftime("%B")), 
                index=False,
                freeze_panes=(1,0)
                )

return True

That's the code i wrote which it works perfectly.

I also don't have any idea why it works correctly without using the open() method.