First of all, window.cardano can be undefined if the user doesn't have a Cardano wallet installed on their browser.

Secondly, you don't need to use await with window.cardano as it's not an async function. Also, you should avoid nested hell and instead try something like this:-

const isCardano = window && window.cardano
const isNamiInstalled = isCardano && window.cardano.nami
const isNamiEnabled = isNamiInstalled ? await window.cardano.nami.isEnabled() : false

First of all, window.cardano can be undefined if the user doesn't have a Cardano wallet installed on their browser.

Secondly, you don't need to use await with window.cardano as it's not an async function. Also, you should avoid nested hell and instead try something like this:-

const isCardano = window && window.cardano
const isNamiInstalled = isCardano && window.cardano.nami
const isNamiEnabled = isNamiInstalled ? await window.cardano.nami.isEnabled() : false

Edit: sometimes window.cardano or window.cardano.nami can be undefined even if you have nami installed. That happens mostly when you try to access these parameters before the extension is even injected to the site. To prevent such scenarios, you can try wrapping your code with a setTimeout delay.

setTimeout(()=> {
    const isCardano = window && window.cardano
    const isNamiInstalled = isCardano && window.cardano.nami
    const isNamiEnabled = isNamiInstalled ? await window.cardano.nami.isEnabled() : false
}, 1500)