SUMMARY An video file (with audio) cannot be placed onto a pair of tracks on the timeline. STEPS TO REPRODUCE 1. Create new project. Place 3 a/v files into the project bin. 2. Add an a/v pair of tracks and drag one a/v file onto the timeline. This works perfectly. 3. Repeat for the second a/v file. Again, this works. 4. Delete the audio track for the second file so now there are two video tracks (V1 and V2) and one audio track (A1). 5. Create a new a/v track pair (V3 and A2). 6. Attempts to drag the third a/v file onto V3/A2 now fail. OBSERVED RESULT See above. EXPECTED RESULT The a/v file should occupy V3/A2. This is fixed by adding in audio tracks until the "A" track number matches the "V" track number. In the example above, add in a new audio track (now A3). The drag operation now succeeds. An alternative solution is to use the project bin clip properties to mute the audio. The video now drops on to V3. This is only suitable if the audio is not needed. SOFTWARE/OS VERSIONS Ubuntu 20.04, Gnome 3.38. KDEnlive 21.04.3 ADDITIONAL INFORMATION
I can confirm this behavior. I'm not sure if this is a bug. In 21.08 it shows on the Timeline bottom right the message "no available track for insert operation". Maybe it would be better you could insert the clip on V3/A2 without message. Workaround: insert the clip on V2/A2 or V1/A1. Hold alt and move the video part of the A/V clip to V3.