[Dúvida] ERRO - no addEventListener

Estou confuso, quando utilizei esse código:

const cep = document.getElementById(cep)

async function buscaEndereco(cep) {
    try {
        var consultaCEP = await fetch(https://viacep.com.br/ws/${cep}/json/)
        var consultaCEPConvertida = await consultaCEP.json()
        if(consultaCEPConvertida.erro) {
            throw Error("CEP NÃO EXISTE!")
        }        
        console.log(consultaCEPConvertida)
        return consultaCEPConvertida;
    } catch (erro) {
        console.log(erro)
    }
}



cep.addEventListener(focusout, buscaEndereco(cep.value))

obtive esses erros:

TypeError: Failed to fetch
    at buscaEndereco (main.js:5:33)
    at main.js:19:34 
    
    main.js:5     GET https://viacep.com.br/ws//json/ net::ERR_FAILED 400 (Bad Request)
    
    index.html:1 Access to fetch at https://viacep.com.br/ws//json/ from origin http://127.0.0.1:5500 has been blocked by CORS policy: No Access-Control-Allow-Origin header is present on the requested resource. If an opaque response serves your needs, set the requests mode to no-cors to fetch the resource with CORS disabled.

Pórem, quando eu usei cep.addEventListener("focusout", () => buscaEndereco(cep.value)) - deu certo, me explica a diferença?