If you use ReactDOM.hydrate to start web application, you will see this warning.

If your application does not use server-side rendering (ssr), please use ReactDOM.render to start.

If you're using Server Side Rendering like NextJS, delete recent code and compare if you've tried to access a variable directly inside of Component scope where DOM is not guaranteed yet. For me, it was:

import { i18n } from 'i18n'

export default function SomeComponent() {
    const initLanguage = i18n.language              <---- causing error

    return ...
}

If you need to access such properties, access it within useEffect, so as to make sure that document is already established by then. It is kinda equivalent to componentDidMount():

import { i18n } from 'i18n'
import { useEffect, useState } from 'react'

export default function SomeComponent() {
    const [initlanguage, setInitLanguage] = useState('en')

    useEffect(() => setInitLanguage(i18n.language), [])

    return ...
}

If your HTML code is like

<table>
  <tr>

you can get this error.
To get around it, use the <tbody> tag like

<table>
  <tbody>
    <tr>

Don't forget to close the tag(s)!