User avatar-ACF fields

Did you try something like this?

User variables

<?php
// Define user ID
// Replace NULL with ID of user to be queried
$user_id = NULL;

// Example: Get ID of current user
// $user_id = get_current_user_id();

// Define prefixed user ID
$user_acf_prefix = 'user_';
$user_id_prefixed = $user_acf_prefix . $user_id;
?>

Display avatar

<?php $avatar = get_field( 'avatar', $user_id_prefixed ); ?>
<?php if ( $avatar ) { ?>
    <img src="https://wordpress.stackexchange.com/questions/304395/<?php echo $avatar["url']; ?>" alt="https://wordpress.stackexchange.com/questions/304395/<?php echo $avatar["alt']; ?>" />
<?php } ?>