I recently went live with a project that does okta integration with mfa. The most common support ticket is people complaining they didn't get their mfa code. Root cause: they never clicked the "send code" button.
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.
While I’m sure your “send code” button was visible, apparently that’s not what they expected. Maybe they expected “Send MFA code”, or “Text me my code”. Maybe the terms you are using don’t match up well with their limited understanding of the domain problem.
Sounds like a problem A/B testing or simple user interviews could help with.