Julie here from Shopify Support.
Where are you located? Because Shopify Payments is only available in select countries, you'll just need to make sure that your business is based in a country supported by Shopify Payments. We actually have a list of countries where Shopify Payments is available here.
If Shopify Payments isn't available in your country, then you will need to select a third-party payment gateway in order to accept credit card payments online. We have compiled a list of gateways available in each country here. If you can let me know where you're located, I'd be happy to share a few recommendations!
As you mentioned that you're currently only using PayPal Express, have you completed the steps necessary for setting up PayPal on your store? Before launching, it's a good idea to ensure that the right PayPal account is connected so you don't experience any delays with receiving your payouts.
Have you already launched your store? If you're still preparing for launch, I recommend checking out our Essential Store Launch Checklist. Aside from setting up your payment gateways, this checklist will help you ensure that you've completed every step necessary prior to launching.
I hope this helps! If you have any further questions, feel free to respond here and I'd be happy to offer further guidance.