Excel file format cannot be determined, you must specify an engine manually.

This error message occurs when you try to read an Excel file using the pandas library in Python, but pandas is unable to determine the file format automatically. To resolve this issue, you need to specify the engine to be used for reading the file.

You can specify the engine by passing the 'engine' parameter to the read_excel() method. For example, if your file is in the older .xls format, you can use the 'xlrd' engine like this:

import pandas as pd

df = pd.read_excel('file.xls', engine='xlrd')

If your file is in the newer .xlsx format, you can use the 'openpyxl' engine instead:

import pandas as pd

df = pd.read_excel('file.xlsx', engine='openpyxl')

Make sure you have installed the necessary engine using pip before using it in your code.