Error HH8: There's one or more errors in your config file: * Invalid value undefined for HardhatConfig.networks.rinkeby.url - Expected a value of t

I get this error when i try to run my script on the rinkeby network:

Error HH8: There's one or more errors in your config file:Invalid value undefined for HardhatConfig.networks.rinkeby.url - Expected a value of type string.

     require('@nomiclabs/hardhat-waffle');require('dotenv').config();module.exports = {  solidity: '0.8.1',  networks: {    rinkeby: {      url: process.env.STAGING_ALCHEMY_KEY,      accounts: process.env.PRIVATE_KEY,    },  },};

Please what could possibly be the problem?