There is no such thing as Tenebrium ability anywhere.
It should be listed in the Abilities tab in the inventory window, under Weapons, between Single-handed and Two-handed, once it is unlocked. It is this ability that is required to add tenebrium damage to a weapon (level 3).
If you check that and still have a problem, please email email@example.com, with a brief description (or link to your post) and the report.zip file (or a Dropbox or Google drive link, etc) generated by the D:OS EE Analysis Tool (for Windows). Extract the zip file somewhere and run the AnalysisTool.exe. When that loads, browse to the install folder (if it is not automatically detected), optionally run the tests first, and then generate a report (it will be saved in the same location as the Analysis Tool).
The report will contain system and game information, as well as your saved games. To reduce the file size, you can delete most of the saves from the zip; leave one before completing Brandon's quest, if available, and your latest (if characters don't have the Tenebrium ability listed, or can not craft at level 3).