The prohibition sign next to a video in the Media Pool are an indication that the video cannot be played. This can be due to the following reasons:
Usage of intermediate video data has been enabled for this video but the data have not been completed yet. Solution: Check whether creation of intermediate display data is listed as a task on the task list. If this is not the case right-click the video in the Media Pool and select Recreate intermediate video data.
No suitable codec has been selected. Solution: In the Video Properties, check whether some other codec can be selected under playback configuration. See Video Properties.
If the field below Playback configuration shows No playback possible you can search for a solution in topic No playback of video possible.