In the example above I check that the Buy button exists by adding a Page content statement and setting the Buy button's CSS selector as the value.
I can now do changes to this element in my code without worrying about it being executed before the element has been loaded/exist.
For example, change the copy of this button:
document.querySelector('.buy-btn').innerText = 'Buy now';
Notice that if you have multiple elements that need to exist before your code executes you need to add separate statements for each to be sure that all exist before the project code executes (on Audience true)