Do FIDE rules allow a player to make notes on the scoresheet like
Here, I wanted to play c4, but thought it would be better to play d4 first.
He played Nc6. I think he should have played Nbd7.
No. Notes could help you during the game, and therefore they aren't allowed. The Laws of Chess say:
First off, I don't actually know whether it is allowed or not, but I have a number of reasons to believe that it may be allowed, or at least be somewhat allowed.
I think there is enough doubt about the interpretation of this rule to require of us to get some clarification. I will most certainly ask an arbiter at my next tournament.
Update: Based on Grizzy's comment below, I am swinging to the camp that thinks it is probably not allowed. At the very least, I would say don't do it unless you have checked with the arbiter (it is probably safest to just not do it though).
It is clearly forbidden by FIDE to write down a game move before playing it on the board. Even though I think there is a difference between writing down moves before you make them and recording (or simply marking) moves that you could have made after the fact, it is probably fair to say that FIDE will find policing your notes too cumbersome. It makes sense to me that they would blankly ban any types of note taking as a matter of practicality.