"Verify it's you" pop-up when I try to make a purchase through Google Play

Whenever I want to make an in-app purchase, a prompt with the header "Verify it's you" with a text box and a submit button appears, but it doesn't say what i need to introduce in the text box. I tried my password, security codes etc, but it doesn't work. I couldn't take a screenshot due to security. Has anyone encountered this issue?