iMovie doesn't display frames or frame rates.
However, here is a link to a frame to time converter so you can precisely locate the specific corrupt frame:
https://www.zapstudio.net/framecalc/
You will need the frame rate of your project to enter into the converter in order for it to make the calculation. Then you enter the frame number given in the error message. The frame rate of your project will be the frame rate of the very first video clip (not photo) that you placed into the newly created project. The converter will give you the time point in the timeline at which the corrupt frame is located. That's the area that you want to scroll around to see if you can find any black frames, white flashes, artifacts, fuzziness, or other corruption that you can remove.
If you know the frame rate, you can just divide that number into the frame number to convert to time. So, if you have a 30 frame per second project, divide 30 into 285 to give you the time point at which frame 285 occurs.
If you don't know the frame rate, , you can expand out your timeline with the slider located above and to the right of
the timeline. Then put your cursor on a clip in the and, while pressing down the r key and pressing down your cursor, slowly drag the cursor along the clip in the timeline. A yellow square will appear as you drag, and a frame counter
display will appear that shows the number of frames as you drag. If the frame counter clicks to 01:00 second right after the reader says 00:29 frames, then you know that the frame rate of your project is 30 frames per second. If the counter clicks to 01:00 second right after the reader says 00:59 frames, then you know that the frame rate of your project is 60 frames per second.
the frame rate into the into the number of frames that, in your case, would be 285 frames, to get the point along the timeline in which frame 285 appears. You will want to scroll a little before and after that point.
-- Rich