The type or namespace name 'System' could not be found (are you missing a using directive or an assembly reference?) fixed

I'm trying to make a web browser application, but when I relaunch Visual Studio Community 2019, it shows me about 500 errors such as The type or namespace name 'System' could not be found (are you missing a using directive or an assembly reference?).

I tried reloading Visual Studio Community 2019 a few times, but still doesn't work either.

Some project infos:

• Target framework: .NET Framework 4.8
• NuGet Packages used: EasyTabs, CefSharp.WinForm

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;
using CefSharp;
using CefSharp.WinForms;
using EasyTabs;