JavaScript Math.random duplicate generation

I have a unique field in a SQL table that currently has 200K rows

I use randomString(6, '0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ') to insert data on that field, I have too many unique conflict error when I want to insert new rows

In my log , I see that randomString generated HEGDDX string today but it generated in 3 months ago too and I have an error on insert

'0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ' has 36 character , I generate 6 length random string, So there is 36^6=2176782336 = 2.17E9 possible cases, So 200K rows in 2 billion has 0.00009 possibility for duplication

Is 0.00009 big enough for too may errors? is Math.random bad random generator? what is alternative for me?

const randomString = function(length, chars) {
  let str = '';
  const charsLen = chars.length;

  for (let i = 0; i < length; i++) {
    str += chars.charAt(Math.floor(Math.random() * charsLen));
  }
  return str;
}