Typeerror A Bytes-like Object Is Required Not Str

How to resolve the error typeerror a bytes-like object is required not str in python?

here is my code which is throwing an error called a bytes-like object is required not str please help me out

for line in lines: 

print(type(line))# <class of 'bytes'> 

if 'substring' in line: 

print('successful')