You can create popup for checkout page..
Please check the URL.
Tira here, from Shopify. Thanks @oscprofessional for the suggestion. I wanted to add my two cents here as well.
It sounds like you want to add a pop-up to your checkout page. I want to let you know that our checkout page's customizations are limited, and adding any coding customizations is available only for Shopify Plus stores. Check out the features of Shopify Plus, here: Shopify Plus plan.
Is there a specific message that you wanted to add to your pop-up?
You can add a pop-up option on your store using a pop-up app from our App Store. Here are some recommendations:
Privy: Pop-Ups, Emails & SMS - You can add spin-to-win wheels, flyouts, email pop-outs.
Sales Pop-up: Social Proof - This app includes a countdown timer, sales notifications and a stock countdown bar.
Email Popup Wheel Game Pop ups - You can create gamified pop-ups.
These apps are developed by third-party developers. I recommend reaching out to the third-party developer for any queries. You can find their contact information under the "Support" section of the app's page.
Here is a list of similar apps you can use: Pop-Up Apps.
Let me know if this helps!