postcss parent selector syntax error

[rokoroku]

.welcome {
  display: flex;
  flex-direction: column;
  justify-content: space-between;
  padding: 0em;
  height: 100%;

  & > div:first-child {
    height: 100%;
    padding: 2em;
    display: flex;
    flex-direction: column;
    justify-content: center;
    text-align: center;

    & > :nth-child(2) {
      font-size: 18px;
      font-weight: 900;
      color: #232323;
      margin: 1em 0;
    }
  }
}

I have no idea why this error occured even the syntax looks valid.

VS code version: 1.21.0-insider
postcss-syntax version: 2.0.0