1. If your product is a variable product, then this happens.

2. Open /includes/template-tags.php and see at function dokan_seller_reg_form_fields around line 367. Remove the seller/custom radio box.

Now add this function to your functions.php, should work.
[php]
function dokan_seller_type_chose_registration() {
?>
<p class="form-row form-group user-role">
<label class="radio">
<input type="radio" name="role" value="customer"<?php checked( $role, ‘customer' ); ?>>
<?php _e( ‘I am a customer', ‘dokan' ); ?>
</label>

<label class="radio">
<input type="radio" name="role" value="seller"<?php checked( $role, ‘seller' ); ?>>
<?php _e( ‘I am a seller', ‘dokan' ); ?>
</label>
</p>
<?php
}

add_action( ‘woocommerce_register_form_start', ‘dokan_seller_type_chose_registration' );
[/php]

Remember, you'll lose the changes once any update comes.