Prioritize by time wasted

Here is the simplest way I know to prioritize usability issues in your report: think about how much time is wasted by that usability issue. E.g.

• An icon was a little ambiguous. It took a couple of seconds for the user to figure it out. - 10 seconds
• The error message only tells that there is a line break error in the CSV file, but it doesn't tell in which row the error is. To correct that line, the user first needs to locate the error, going row-by-row. - 1 hour.

The latter issue is way more important from a usability standpoint.

If you happen to know the frequency of these interactions (e.g. 10.000 people clicked the icon and 100 people ran into the line break error) you can make an even better priority estimation.

icon - 10.000 * 10 seconds = 100k seconds wasted

error location - 100 * 3.600 seconds = 360k seconds wasted