5th dnd – Can a 1st level character have a capacity score greater than 18?

I was playing with a DM saying that the rules state that you can not have a 1st level character with a capacity score greater than 18. I did not want to argue, so I did. I do not find this rule anywhere.

We use the method "roll 4d6 for each ability score, let go the lowest" to generate capacity scores.

Is there a rule stating that a 1st level character can not have a capacity score greater than 18?