Learn Italian: Il medico avrà il referto tra due ore. - The doctor will have the medical report in two hours.

In Italian, tra (and fra) indicates the amount of time that will pass before an event happens or starts. The doctor has to wait two hours, and then he will get the report. If you used in due ore, it would mean the action itself takes two hours to complete from start to finish, which doesn't make sense for simply receiving or possessing a document.

Is there any difference between tra and fra? Could I say fra due ore?

You can absolutely say fra due ore. Tra and fra are identical in meaning and usage. Native speakers choose between them purely for sound flow, usually to avoid repeating the same starting letters of the next word (such as avoiding tra tre giorni and saying fra tre giorni instead).

What exactly is a referto? Can I use it for a business report?

No, referto is specific to medicine. It means a medical report, test result, or doctor's written diagnosis. For a business, academic, or news report, you would use words like rapporto or relazione.

Could avrà be the future of probability in this sentence, meaning 'the doctor probably has'?