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

This error message usually occurs when you are trying to read an Excel file using Pandas and it cannot determine the file format automatically. To resolve this issue, you need to specify the engine parameter when reading the file.

For example, if you are using the read_excel() function in Pandas, you can specify the engine parameter as 'openpyxl' or 'xlrd' depending on the version of Excel file you are reading.

Here is an example code snippet:

import pandas as pd

# Reading an Excel file using the openpyxl engine
df = pd.read_excel('file.xlsx', engine='openpyxl')

# Reading an Excel file using the xlrd engine
df = pd.read_excel('file.xls', engine='xlrd')

Note that the 'openpyxl' engine supports reading both .xlsx and .xls file formats while the 'xlrd' engine only supports reading .xls file format.