vue: How to get variable value in nested function?

Question

I can't get variable "account" in this Vue.js source in a part.

    export default {
      
      data() {
        return {
          visible: true,
          account: '',
          password: '',
          sso: '',
        }
      },
      methods: {
        submit() {
          if (this.account == '') {
            return alert('Please enter your ID.')
          } else if (this.sso==0 && this.password == '') {
            return alert('Please enter your Password.')
          }
    
          let data = {
            account: this.account,
            sso: this.sso,
          }
          if(this.sso == 0 ){
            data['password'] = sha256(this.password)
          }else{
            data['password'] = sha256('Demeter_'+this.account)
          }
          createLogin(data).then((res) => {
            console.log('createLogin :: res', res)
            if (res.status === 200) {
              if (res.data.status == 409) {
                return alert(res.data.message)
              }
              if(this.sso == 1){
                Kakao.Auth.login({
                  success: function (response) {
                    console.log("Kakao.Auth.login - success response ->", response);
                    Kakao.API.request({
                      url: '/v2/user/me',
                      // property_keys: [
                      //   'properties.nickname'
                      // ],
                      success: function (response) { //'success function'
                        console.log("/v2/user/me - response.id ->"+ response.id)
                        console.log("data ->", data)
                        let data = {
                          accesstoken: res.data,
                          account: this.account, // account : undefined
                        }
                        console.log("/v2/user/me - data ->", data )
                        this.$store.dispatch('store/login', data)
                      },
                      fail: function (error) {
                        console.log(error)
                        return alert('Login failed')
                      },
                    })
                  },
                  fail: function (error) {
                    console.log(error)
                  },
                })
                
              }else{
                let data = {
                  accesstoken: res.data,
                  account: this.account, // account : admin
                }
                console.log("else - data ->", data )
                //this.$store.dispatch('store/login', data)
              }
            } else {
              let message = res.data.message
              alert(message)
            }
          })
        },
      }
    }

When I uncheck sso(sso = 0), then it shows this.account as which is written in the account input box. But if I check sso(sso = 1), then it showed this.account as undefined. I don't know why this happens. Is the reason that function is nested so? Then how can I use that account value in that 'success function'?