Your function prints your string to stdout, then returns the list itself unchanged.

Build a string and return that instead of printing:

def to_string(my_list):
    result = ''
    last = len(my_list) - 1
    for pos, elem in enumerate(my_list):
        result += str(elem)
        if pos != last:
            result += ', '
    return result

This loops over all elements, keeping a position counter with the enumerate() function; this way it's easy to detect if we are adding the last element to the result.

but I am unable to print it as a string

Just unpack the arguments out of the list using the * operator when calling print() and use "" as a separator:

>>> L = ['g', 'i', 'n', 'o', 'r', 't', 'S', '1', '3', '2', '4']
>>> print(*L, sep="")
ginortS1324

Use ''.join:

xs = ['1', '2', '3']
s = ''.join(xs)

If the list contains integers, convert the elements to string before joining them:

xs = [1, 2, 3]
s = ''.join(str(x) for x in xs)

','.join(f'{x} {y}' for (x,y) in l)

You have two processes you need to combine. First you need to unpack each pair of co-ordinates into a string like 0 1 for [0, 1] this could be done using a join but, IMO, it's easier and clearer to simply used a formatted string, that's the f'{x} {y}' which replaces the {x} and {y} with the value of the variable x and y.

Second, you need to read these pairs out of the containing list l, this can be done using a comprehension. In your case you want to unpack as you go which you can do with for (x, y) in l. You can then combine these with a join to introduce the commas between co-ordinates that you want.

So, including your encasing parentheses we end up with a function definition of:

def conv(l):
    return f"(({','.join(f'{x} {y}' for (x,y) in l)}))"

join method of strings.

','.join(test)

Although, since you've got integers in that list, you'll have to convert them to strings.

','.join(str(num) for num in test)