Copyright - Youtube

Anyhow scores created with MuseScore are as much yours to use as scores you create with pencil and paper. ...if you own the rights to the music itself - your original composition, a public domain source, or something you have legal permission to use in this way - then you are free to sell the sheet music you create. But, just as with pencil and paper, the mere fact that you created the physical score doesn't automatically give you the rights to the music on it if you didn't own that already.
So the first question you have to ask yourself is: Do I have copyright or consent from the rights holder?
Also see: https://musescore.org/en/node/325857

MuseScore in no way whatsoever affects anything having to do with copyright. The question of whether or not you have the rights to create or publish an arrangement is exactly whether you use MsueScore or whether you pencil and paper. The answer doesn't depend on the software you use, it depends on whether the piece you are arranging is under copyright protection in your particular country or not. If it is not in copyright, you can do anything you want with it. if it is in copyright, your right are more limited, but again, they vary by country. In general, you can usually create the arrangement but not publish it (including on YouTube) without permission. YouTube has license agreements with tons of copyright owners that do give you permission; YouTube will simply put an ad on your video to pay the copyright holder.