Here is a very useful and simple script that lists all the available networks in your HRE. Run it as a Hardhat script to see if Sepolia is listed there at all:

const hre = require("hardhat");

async function main() {
  console.log("Available networks in Hardhat configuration:");
  console.log(hre.config.networks);
}

main()
  .then(() => process.exit(0))
  .catch((error) => {
    console.error(error);
    process.exit(1);
  });

It should list your Sepolia network like this: