@WebElaine is correct. I had to use the wp_get_current_user(); and remove global.
This is what worked for me
function download_to_wallet() {
$current_user = wp_get_current_user(); // get current user's information
$qr_url = $current_user->qrbase_code_pass_url;
return '<a href="'. $qr_url . '"><img class="qr_download" src="https://example.com/wp-content/uploads/2018/08/add-to-apple-wallet-logo.png" style="width:200px;"></a>';
}