how to print first list element inside a list of lists in a for-loop structure?

If you print codes_chunks:

print(codes_chunks) # [['A1', 'A2'], ['A3', 'A4'], ['A6', 'A7']]

Not you're selecting code_cunks[0] which is ['A1','A2'].

If you now iterate over this you'll get A1 which is required but you're iterating again which results in A & 1. So, you need to remove the extra loop.

Correct code for desired output:

########## define a split function ########

def chunks(lst, n):
    """Yield successive n-sized chunks from lst."""
    for i in range(0, len(lst), n):
        yield lst[i:i + n]

########## splitting the list #############


countries = ['Spain', 'Portugal']
codes = ['A1', 'A2', 'A3', 'A4', 'A6', 'A7']

codes_chunks = list(chunks(codes, 2))
###########################################

# tried to print just for the first list #

for country in countries:
    for chunks_sample in codes_chunks[0]:
        print(chunks_sample+'-'+country)
###########################################

TL;DR:

Looping over codes_chunks[0] already provides the "code" A1 and A2, thus your innermost loop is just one layer too deep.

Step by step:

>>> codes_chunks = list(chunks(codes, 2))
[['A1', 'A2'], ['A3', 'A4'], ['A6', 'A7']]

Thus, codes_chunks[0] in your second-order loop is only ['A1', 'A2'].

>>> for country in countries:
...     # code_chunks[0] = ['A1', 'A2']
...     for chunks_sample in codes_chunks[0]:
...         # you don't need another loop here
...         # chunks_sample = 'A1' or 'A2'
...         # `for code in chunks_sample` would loop over 'A1' yielding 'A' and '1'
...         print(chunks_sample+'-'+country)
A1-Spain
A2-Spain
A1-Portugal
A2-Portugal

Generalization to more chunks

If you need to generalize your loops to all codes_chunks, you have two options:

Option 1: Wrap another for-loop around everything else, to alternate the countries in the output:

>>> for codes_chunk in codes_chunks:
...    for country in countries:
...        for chunks_sample in codes_chunk:
...            print(chunks_sample+'-'+country)
A1-Spain
A2-Spain
A1-Portugal
A2-Portugal
A3-Spain
A4-Spain
A3-Portugal
A4-Portugal
A6-Spain
A7-Spain
A6-Portugal
A7-Portugal

Option 2: Add the same loop inside your outermost loop to complete countries one-by-one:

>>> for country in countries:
...    for codes_chunk in codes_chunks:
...        for chunks_sample in codes_chunk:
...            print(chunks_sample+'-'+country)
A1-Spain
A2-Spain
A3-Spain
A4-Spain
A6-Spain
A7-Spain
A1-Portugal
A2-Portugal
A3-Portugal
A4-Portugal
A6-Portugal
A7-Portugal