If you are using Vite, use import.meta.env instead, process.env is removed.
And make sure variables start with VITE_ in .env file.
//in .env
VITE_GITHUB_URL= 'https://api.github.com'
//in App.jsx calling environment variable
console.log(import.meta.env.VITE_GITHUB_URL)
//never use sensentive data in .env as vite or create react app include in build
//instead create server.
process is a global object in Node.js that provides information about,
and control over, the current Node.js process. It is not available in the browser.
If you are using Vite, a web development build tool, you will not have access
to the process object because Vite is designed to run in the browser and does
not have access to the Node.js runtime.

import { defineConfig } from 'vite';
import react from '@vitejs/plugin-react';
export default defineConfig({ plugins: [react()], server: { host: '0.0.0.0', }, define: { global: {}, 'process.env': {} },
})
// add global: {} and 'process.env': {}

import { defineConfig } from 'vite'
import react from '@vitejs/plugin-react'
// https://vitejs.dev/config/
export default defineConfig({ plugins: [react()], server: { host: '0.0.0.0' }, define: { 'process.env': {} }
})

{"tags":[{"tag":"textarea","content":"# WITH VITE import.meta.env.VITE_API_KEY_X\n# WITH REACT CREATE process.env.REACT_APP_API_KEY_X\nFROM DOTENV SPECIFICS DEFINITIONS","code_language":"javascript"},{"tag":"p","content":"Uncaught ReferenceError: process is not defined react"},{"tag":"p","content":""}]}