The problem was arising because the function was running before the initialization can be done and that is why I modified the code as follows.

var account;
window.addEventListener('load', async () => {
    if (window.ethereum) {
        window.web3 = new Web3(ethereum);

        ethereum.autoRefreshOnNetworkChange = false;

        const accounts = await ethereum.enable();
        account = accounts[0];
        console.log(account); //here the account working properly.
        get_request_details();
    }
});

var contractaddress = '0x26708Df214A65Dda444E61266642e6650F4e8923';

function get_request_details() {
    console.log(account); //here it is also working
}

I called the function from the async event only so that when the initialization will be done after than only the function should run.

I don't see the contractaddress variable used anywhere. So here is my opinion.

const get_request_details = function(account) {
    console.log(account);
}

const run_after_window_loaded = async function() {
    if (!window.ethereum) {
        return false;
    }
    window.web3 = new Web3(ethereum);
    ethereum.autoRefreshOnNetworkChange = false;

    const accounts = await ethereum.enable();
    console.log(accounts);

    if(accounts.length > 0){
        get_request_details(accounts[0]);
    }
}

window.addEventListener('DOMContentLoaded', async function(event) {
    await run_after_window_loaded();
});