You use of a lot of 'you cannot'. This may be the case in game that you are running, but you should understand that not everyone thinks the same as you do.
The assumption of the starting section is that the character starts at Level 1. Only later does the game talk about starting at higher levels. No, it does not explicitly state that the piece on choosing a background is only to be used for characters of a particular starting level, but then it doesn't need to. It is pretty obvious that the advice is there for starting players and starting characters, and the default assumption is starting at Level 1.
Please point me to the section in the PHB where this is stated, otherwise it is just your opinion. This is the 5th edition of a system that has existed for decades, it should not be so difficult to find evidence if this is the case
Seriously? You want absolute proof that the introductory section of the book where it guides you through creating you first character, explaining what a character sheet is and what the various heading are, is aimed at starting players? Or that the default assumption is that they start at level 1? You have gone beyond any kind of reasoned argument here.
No, there is not large-type heading stating that Chapter 1 of the Player's Basic Rules is designed to help new players create a character. There is, however, a very strong suggestion backed up by the language used, the way that everything is carefully explained, the fact that it is introducing readers to the game, and the fact that it is standard practice. Yes, D&D5e is the 5th edition. I have played all the others and all the other editions had similar introductory chapters which spoon-fed you through character creation. All were aimed at new players. All assumed starting a character at Level 1 for new players.
