people who have no aptitude for chess

What are the signs that people have no aptitude for chess?
What if, despite learning about chess in various ways (books, videos, puzzles, etc.), they remained at a number like 500 ELO for several months (or over a year?)?

I wouldn't convince them to quit chess, but
It is clear that they are not naturally suited to chess and are wasting their efforts.

@1

"What are the signs that people have no aptitude for chess?" ++ Other interests.

"What if, despite learning about chess in various ways (books, videos, puzzles, etc.), they remained at a number like 500 ELO for several months (or over a year?)?"
++ They do not do the right thing. Blunder checking, lost game analysis...

"I wouldn't convince them to quit chess" ++ If they have no fun, then they should quit.

"It is clear that they are not naturally suited to chess"
++ Every human is naturally suited. It is a matter of dedication or lack of it.

"wasting their efforts" ++ Rather doing the wrong efforts.

If you're still of school age then your aptitudes haven't stabilised yet, by the time you leave school you should have a pretty good idea what you are and aren't good at.

Just as some people are born with the ability to play musical instruments or figure out math problems without years of practice so some people are born with the natural ability to play chess far above the average human. To deny this is to deny reality. Its silly to even argue about. Everyone can't become a strong player but everyone can enjoy the game no matter what level they are at.

@5

"some people are born with the ability to play musical instruments or figure out math problems without years of practice so some people are born with the natural ability to play chess"
++ No, this is false. The pedagogue Lazlo Polgar claimed he could make any child excel at either chess, math, or classical music, provided starting at young age. He proved that with his 3 daughters.

"Everyone can't become a strong player" ++ Everybody can, provided starting young.

"everyone can enjoy the game no matter what level they are at"
++ That is true. Sometimes the low levels enjoy chess more than the high levels.