SUMMARY
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In a simple project, with 2 video tracks (one of them with images), 1 empty audio track, I try to drop a video a have yet imported to KDEnlive, however, just when my mouse reach the empty tracks, KDEnlive changes mouse pointer to "Prohibited" icon, a error sound is played and a message with red background appears with the text: "No track available for insertion operation" (translated from Spanish).
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STEPS TO REPRODUCE
1. Select the video track
2. Drag it with mouse to empty track

OBSERVED RESULT
KDEnlive changes mouse pointer to "Prohibited" icon and an error sound is played and a message with red background appears with the text: "No track available for insertion operation" 

EXPECTED RESULT
To insert video in any empty track

SOFTWARE/OS VERSIONS
Linux, OpenSUSE, Plasma

ADDITIONAL INFORMATION
The issue is related to have AUDIO in the video to be inserted. In my user opinion, message should be changed, warning about user need an AUDIO track for that video not simply warn about a "track" is required.