WordPress does not allow uploading of SVG by default because of security risks.
You can allow SVG to be uploaded on your website by using a plugin like:
Otherwise you can just add this code to your functions.php:
WARNING: Before doing the below, please make sure to make a backup, since your website can potentially crash if you do this incorrectly.
$new_filetypes = array();
$new_filetypes['svg'] = 'image/svg+xml';
$file_types = array_merge($file_types, $new_filetypes );