Problem with function

I'm using vim 8.1.682 on Windows.
I'd like to know what is wrong with following function?:

function! Finalize()
:'<,'>join
:s/ /,/g
endfunction
vmap ,f :call Finalize()<cr>

If I visually select the following text:

2.233 2.204
2.077 2.066 1.987
6.381 6.372 6.427 6.437
6.152 6.115

It correctly executes, but I get the error:
E16: Invalid range

But if I execute separately each line inside the function, i.e.
:'<,'>join
:s/ /,/g

I don't get any error

Thanks in advance,

--
Cesar

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