When interacting with a web page, Load Tester has many of the same limitations that a human user has. Among those limitations is that Load Tester cannot click (or type text on) an element that is invisible, disabled, etc. Thus, if your testcase encounters an error indicating this fact, there are two common causes to investigate – the testcase is missing a step or the testcase is proceeding too fast.
Would a human user need to do something else before the element can be clicked? See Element Not Ready Yet for an example and solution.
Load Tester’s virtual user can execute a testcase much faster than a human user. Tiny delays in the page load (such as some AJAX enabling a link or button) that are imperceptible to a human can bring a testcase to a screeching halt. In this case you might find that the element is visible in the screenshot, because the element became visible (or enabled, etc) between the time the step action was executed and the time the screenshot was captured. For an example and solution, read Testcase Proceeding Too Fast.