It's not clear whether you intend to blame the user or the tool, but my inclination, absent any other information, would be to blame the tool.

Maybe they clicked forward from a previous screen and were presented with an active text field and the "send code" button, but didn't recognize it as a button, or misunderstood its purpose, expecting the code to be delivered automatically, or thinking that it meant "send code from the user to the website" (i.e., after they have typed it in).

Whatever the details, a bunch of users have an expectation that doesn't match the tool's reality. It's the job of the designer to recognize or discover that gap and find ways to remedy it.