Error: too many values to unpack

[buran]

one way is to add catch all variable

with open('input.txt') as f,open('total.txt', 'w') as f_out:
    header = next(f)
    c1, c2, *_ = header.strip().split(' ')
    f_out.write(f'{c1}  {c2}    Sum\n')
    for line in f:
        line = line.strip()
        num1, num2, *_ = line.split(',')
        num1, num2 = int(num1), int(num2)
        num3 = num1 + num2
        f_out.write(f'{num1}    {num2}  {num3}\n')

*_ will catch all remaining values being unpacked. underscore is a valid variable name used to denote its a throw away variable, but you can use any name you want

another way is to use slicing when unpacking

c1, c2 = header.strip().split(' ')[:2]

Then same for num1 and num2 unpacking

another way to process altogether the file is to use csv module and csv.DictReader and csv.DictWriter which will simplify the code.

Note that your example input file has no commas, although you split your line at commas. csv module will not work if the header and the data rows have different separator