Why is a suspicious popup appearing on my cart page?

[digidoers]

The website was functioning properly three to four days ago; however, a popup began to appear on the cart page suddenly. When we click on the 'Proceed to Checkout' button on the cart page, the suspicious popup appears. Despite our efforts, we have been unable to identify the code responsible for this issue. The current theme is custom-made, and we even attempted to install the Dawn theme, but the popup persists. Notably, the popup does not appear in the theme editor. Any assistance from someone would be greatly appreciated. The website URL in question is https://www.krudofish.com/