Why can't I harvest canticle bark?

Yes, this is a known bug. This is caused by a scripting error within the game. There are several things you can try, in order to fix this issue.

1. Try exiting, and re-entering the glade to see if it fixes.

2. Try fast traveling to a far off location, then travelling back into the area, and see if it works.

If none of this works, then there is a more direct approach to get around this.

Solution: Use the console commands: 'player.additem xx0191b7 7' (to add the Canticle Barks) then: 'setstage DLC1VQ06 30' (next stage of the quest), then just continue the quest, where "xx" is the load order for the DLC Dawnguard (typically 02).

This is a guaranteed fix, and doing this should allow you to continue your game and circumvent the bug.

Good luck!

Edit: In regards to "not wanting to add them to your inventory" that is the only option, if the other things do not work as mentioned above. It's the only known solution to the problem. However, I am not 100% sure, but it is a possibility that one of the unofficial patches or bug fix mods might have fixed this issue at some point, that is a high possibility. There are a few unofficial mods that are said to fix many known bugs, and many issues, errors, and scripting issues. There is a strong possibility that one of these mods would fix this issue you are encountering. For example, unofficial skyrim patch, or misc fixes patch, both found on Nexus Mods.

Edit 2: I also just noticed, since it's a Dawnguard quest, the "Dawnguard Unofficial Patch" would be another patch mod to look into as well.

Edit 3: I just did several searches for mods on Google, Nexus Mods, and so forth. I also did several searches through the Unofficial Dawnguard Patch, and it doesn't seem like anyone has tackled the scripting issues for this quest via any known mods. So the only option it would seem is to use the command console, to do as was described above.

Edit 4: I might have a different solution. I did some further digging, and found this: http://www.uesp.net/wiki/Skyrim_talk:Unseen_Visions Specifically the excerpt here: "Simply put, I grabbed the draw knife and activated the tree only to go into 3rd person view and watch myself do nothing for about 4 seconds. Looked up this page and tried reloading and deactivating my mods as it suggested and nothing worked. Spent 15 minutes on google only to find the solution mentioned once. That being that its waiting for you to preform a resorse gathering animation such as chopping wood, after using tcl consol command to escape the cave I tried this and it worked. immidately after I finished my wood chopin I recived the bark and a quest update. — Unsigned comment by 99.235.239.235 (talk) at 04:30 on 14 August 2012 " Below that, two different people confirm this fix worked. This might be a way to get a solution, and not have to "add item" which you said you would rather not do. Try it out, and let me know if it worked.