how to add radio field on user meta on function.php?

You are using the wrong name on the radio inputs, radio-inline but when you try to get that value you are using $_POST['gender']

I’ve updated you code and made some improvement too, with the checked() function

function show_extra_profile_fields( $user ) { 

    $gender = get_user_meta( $user->ID, 'gender', true );
?>

<section>
    <label class="label" for="gender">Gender</label>
    <div class="inline-group">
        <label class="radio"><input type="radio" name="gender"  <?php  checked( $gender, 'Male' ); ?> value="Male" /> <i>Male</i></label>
        <label class="radio"><input type="radio" name="gender"  <?php  checked( $gender, 'Female' ); ?> value="Female" /> <i>Female</i></label>
    </div>
</section>

 <?php }

OBS: you should sanitize data too.

OBS2: Do what @kaiser said:

Questions are meant to be references for later visitors as well. Please rework your questions spelling/punctuation/grammar/capitalization so it as as easy to read as possible. Thanks.

Sorry for bad english

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