Decode it.

>>> b'a string'.decode('ascii')
'a string'

To get bytes from string, encode it.

>>> 'a string'.encode('ascii')
b'a string'

Your bytes simply cannot be decoded as utf-8, just as the error message tells you.

utf-8 is the default encoding parameter of decode - and the best way to put in the correct encoding value is to know the encoding - otherwise you'll have to guess.

And guessing is probably what the website does, too, by trying the most common encodings, until one does not throw an exception:

def decodeAscii(bin_string):
    binary_int = int(bin_string, 2);
    byte_number = binary_int.bit_length() + 7 // 8
    binary_array = binary_int.to_bytes(byte_number, "big")
    ascii_text = "Bin string cannot be decoded"
    for enc in ['utf-8', 'ascii', 'ansi']:
        try:
            ascii_text = binary_array.decode(encoding=enc)
            break
        except:
            pass
    print(ascii_text)

s = "01010011101100000110010101101100011011000110111101110100011010000110010101110010011001010110100001101111011101110111100101101111011101010110010001101111011010010110111001100111011010010110110101100110011010010110111001100101011000010111001001100101011110010110111101110101011001100110100101101110011001010101000000000000"
decodeAscii(s)

Output:

S°ellotherehowyoudoingimfineareyoufineP

But there's no guarantee that you find the "correct" encoding by guessing.

You are looping over the individual characters of the input message, but you need to instead look for groups of 9 characters (2 times 4 binary digits and the space). Your mapping has keys like '0100 1001', not '0' and '1' and ' '

The simplest approach (albeit a bit brittle) would be to loop over indices in steps of 10 characters (1 extra for the space between the characters), then grab 9 characters:

for i in xrange(0, len(messageDecode), 10):
    group = messageDecode[i:i + 9]
    print inverseBINARY[group],    

The xrange() object produces integers 10 apart; so 0, 10, 20, etc. The messageDecode string is then sliced to grab 9 characters starting at that index, so messageDecode[0:9] and messageDecode[10:19], messageDecode[20:29], etc.

A more robust approach would be to remove all spaces and grab blocks every 8 characters; that'd leave room for extra spaces in between, but you do have to re-insert that space to match your keys:

messageDecode = messageDecode.replace(' ', '')
for i in xrange(0, len(messageDecode), 8):
    group = messageDecode[i:i + 4] + ' ' + messageDecode[i + 4:i + 8]
    print inverseBINARY[group],    

or you could perhaps not include the spaces in your inverseBINARY mapping here:

inverseBINARY = {v.replace(' ', ''): k for k, v in BINARY.items()}

and then simply slice every 8 characters:

messageDecode = messageDecode.replace(' ', '')
for i in xrange(0, len(messageDecode), 8):
    group = messageDecode[i:i + 8]
    print inverseBINARY[group], 

def msgencoder(msg):
    return format(int(bytes(msg, 'utf-8').hex(), base=16), 'b')

def msgdecoder(msg):
    return bytes.fromhex(format(int(msg, base=2), 'x')).decode('utf-8')

a = msgencoder('hello world')
b = msgdecoder(a)

print(b)

hello world