Function that adds two items from a list in search of those that will match some given sum
def main():
    k = 21
    listA=[10,11,3,7]
    print (k,' = '," + ".join([str(i and e) for i in listA for e in listA if k==(i+e)]))

main()


This minor change to your code will do what you're after.  Put a " + ".join( ) around your list creator and cast the (i and e) items to str, that will then join all string elements in the list together into a string with + as the separator (there are spaces in there, yes, and this is necessary to get the output you're after).

Proof of Concept (via tio.run) that shows you how this will execute.

Note I also made a change to add spaces around the equals sign, indentation, and extra spacing for readability, as well as indentation standardization (4 spaces for a single indentation level).