Upgrade to PRO Account
Although free account allows you to create sitemap for your website, the upgraded account provides much more benefits, please check the Free vs Paid comparison table.
You would select the subscription plan depending on the number of pages on your website (there are monthly as well as yearly subscription options), then select the payment method (using the credit/debit card via Stripe payment service or using your Paypal account).
After the payment is done your account will be upgraded immediately and PRO Sitemaps will start creating new sitemaps according with your new subscription status.
Note that you can upgrade your account to a different plan at any moment. We will refund the remaining part of your current subscription plan when your account is upgraded.