get this error

on this code

"use client"
import posthog from "posthog-js"
import { PostHogProvider } from "posthog-js/react"

if (typeof window !== "undefined") {
    posthog.init(process.env.NEXT_PUBLIC_POSTHOG_KEY!, {
        api_host: process.env.NEXT_PUBLIC_POSTHOG_HOST
    })
}

export function PHProvider({ children }: { children: React.ReactNode }) {
    return <PostHogProvider client={posthog}>{children}</PostHogProvider>
}

I don't get why, it's from official docs of vercel

https://vercel.com/guides/posthog-nextjs-vercel-feature-flags-analytics