PropertyChanged remains null

[Evanna456]

class file
what's wrong with my code?, why PropertyChanged doesn't invoke and remains null.

[ScarletKuro]

It actually works fine and getting invoked during debugging.
Or do you mean why the TextContent is not updated with indexViewModel.TextContent = "asdasdadasd"; from your Generate class?
Because you initialize new instances of IndexViewModel inside the Generate class instead of reusing the existing VM. I would also add, that your ICommand implantation is wrong and I would recommend you to use the RelayCommand from the CommunityToolkit.Mvvm that you have installed.
Here is a full working code.

Code under this spoiler,

using System.ComponentModel;
using System.Runtime.CompilerServices;
using System.Windows;
using System.Windows.Input;
using CommunityToolkit.Mvvm.Input;

namespace WPF_Password_Generator.ViewModels
{
    public class IndexViewModel : INotifyPropertyChanged
    {
        private readonly Models.Index _indexModel = new Models.Index();

        public string TextContent
        {
            get
            {
                return _indexModel.textContent;
            }
            set
            {
                _indexModel.textContent = value;
                OnPropertyChanged(nameof(TextContent));
            }
        }

        public ICommand GenerateCommand { get; set; }

        public IndexViewModel()
        {
            GenerateCommand = new RelayCommand(OnGenerateCommand);
        }

        private void OnGenerateCommand()
        {
            MessageBox.Show(TextContent); //Show current value
            TextContent = "asdasdadasd"; //Replacing value, is this intended?
            MessageBox.Show(TextContent); //Show replaced value
        }

        public event PropertyChangedEventHandler? PropertyChanged;
        protected void OnPropertyChanged([CallerMemberName] string? propertyName = null)
        {
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
        }
    }
}

A few more notes: Since you are using CommunityToolkit.Mvvm You should use what the library offers you. For instance, instead of implementing your own INotifyPropertyChanged for ViewModels, you can use ObservableObject. Sometimes it's even acceptable to use it in your Model(MSDN link). And so on, this will help you to make the code more clean.
Example:

Code under this spoiler,

ViewModel

    public class IndexViewModel : ObservableObject
    {
        public Models.Index IndexModel { get; }

        public ICommand GenerateCommand { get; set; }

        public IndexViewModel()
        {
            IndexModel = new Models.Index();
            GenerateCommand = new RelayCommand(OnGenerateCommand);
        }

        private void OnGenerateCommand()
        {
            MessageBox.Show(IndexModel.TextContent); //Show current value
            IndexModel.TextContent = "asdasdadasd"; //Replacing value, is this intended?
            MessageBox.Show(IndexModel.TextContent); //Show replaced value
        }
    }

Model

    public class Index : ObservableObject
    {
        private string _textContent = string.Empty;

        public string TextContent
        {
            get => _textContent;
            set => SetProperty(ref _textContent, value);
        }
    }

Xaml

 <TextBox Grid.Row="0" 
               Grid.Column="2"
               HorizontalAlignment="Stretch"
               VerticalAlignment="Center"
               Margin="10"
               Text="{Binding IndexModel.TextContent, UpdateSourceTrigger=PropertyChanged}"

[Evanna456]

Thanks a lot. :D