I'm pretty sure that you can't copyright mere chords.
If only it were that simple!
For songs there are three potentially protectable elements:
1. The lyrics;
2. The music; and
3. Any recorded performance (this is not technically protected by copyright, but by a "neighbouring" performance right).
Each part is protected if it's the author's (performer's) original creation.
Copying a *substantial* part without permission infringes the right. For most songs (12 bar blues aside) the complete chord sequence is a substantial part. Where songs share the same chord sequence (eg 12 bar blues), we get into an argument about originality and copying.
Added to that there are exceptions to the rights (fair use in the US, private use in Civil law countries), which never tell you with certainty if they apply. Oh, and the law which applies is the law of the country where you infringed the right (which, on the internet, includes communication to the public, so not necessarily the law of the place where you made your YouTube recording or where YouTube's servers live).
You might reasonably think this is nonsensical and you'd be right. Modern copyright law began with an English Act of 1611 designed to protect printers, and has been extended piecemeal ever since to cover plays, operas and musicals, printed music, piano rolls, gramophone records, radio broadcasts, TV broadcasts, cable broadcasts, satellite broadcasts and finally the internet. I probably missed some things from that list. Hardly ever is anything taken away.
For practical purposes the following strategy is pretty safe for private individuals:
1. Don't copy anything if you think the creator would object.
2. If the creator does object, say sorry nicely and stop your copying.
Also be aware that the creator (who might like your cover version) has probably assigned the rights to a publisher (who might not). But I know of no case, anywhere in the world, of an amateur cover version resulting in legal action (as opposed to threatening letters).