Element with locator not found in page pop up
First of all, if you're checking for an element within a POP-UP windows, first you have to select the newly popped-up window with "Select window".After this is covered, try checking if the page contains the element.If it does, try checking for any visibility attribute that could prevent it from being displayed on the screen.A transparent object sitting on top of the searched object could also make the visibility check fail (imho visibility checks on Robotframework are a really obscure matter).
For debugging, add a capture page screen shot just after the sleep and check yourself visually for the state in which the page finds itself at that moment.