Hello,
When I want to add music to an audio track, I have the forbidden meaning.
Is this the format of my music that is in wma?
Thank you for your help.
If you could put the answer in French, it would be great for me.
Save, Close, Re-open.
This has been mentioned many times on the forum 
Or use this tools :
Add, Insert, copy, paste…
(Ajouter, Copier, coller, insérer…)
When the often encountered ‘Forbidden’ icon appears, nothing will add/insert/paste the already imported file…
However, this issues is solved by saving the project, closing Shotcut then re opening the saved file (.mlt).
It has come up umpteen times on this forum. I guess it’s a bug that hasn’t been squashed yet.
Yea I’ve had this problem too. Re-opening fixes it for me every time.