Enable/Disable textBox when a certain element of the wpf combobox is selected

As Leodev says in the comments, there are several ways to fix this problem, one of the easiest is to use a boolean.

To explain it to you, I will use an example.

Assuming we have the following ViewModel

public class TestViewModel : INotifyPropertyChanged
{

public event PropertyChangedEventHandler PropertyChanged = (sender, e) => { };
protected void OnPropertyChanged() => PropertyChanged(this, new PropertyChangedEventArgs(null));
/// <summary>
/// Lista de ejemplo..
/// </summary>
public IEnumerable<int> Lista { get; } = new List<int>{1,2,3,4,5};
/// <summary>
/// Variable privada elemento seleccionado..
/// </summary>
private int mNumeroSeleccionado;
/// <summary>
/// Exposición del elemento seleccionado
/// </summary>
public int NumeroSeleccionado
{
    get => mNumeroSeleccionado;
    set
    {
        mNumeroSeleccionado = value;
        //Disparo OnPropertyChanged
        OnPropertyChanged();
    }

}
/// <summary>
/// Booleano, que devolverá verdadero, si el número seleccionado es 2 o 4.
/// </summary>
public bool TextBoxActivo => NumeroSeleccionado == 2 || NumeroSeleccionado == 4;
}

Then in xaml

<ComboBox 
            ItemsSource="{Binding Path=Lista}"
            SelectedItem="{Binding Path=NumeroSeleccionado}"></ComboBox>

        <TextBox IsEnabled="{Binding Path=TextBoxActivo}"></TextBox>

In this way, when changing the option in the ComboBox, the Setter will be executed NumeroSeleccionado, therefore, the method OnPropertyChanged, which will update our view, will be executed.

When executed, the property will be recalculated TextBoxActivo, which is bound to IsEnabledthe TextBox.

Therefore, if 2 or 4 is selected in the Combo, the TextBox will be active, otherwise, it will not be.

Let's see... I did this to the eye... According to my comment, what I would do is something like this:

First: Create a Converterthat receives the SelectedItem(the entire object)

//using System.Windows.Data;

namespace MyProject.Converters
{
    public class TipoToBoolean : IValueConverter
    {
        public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
        {
            if (value != null && value.GetType() == typeof(TipoPpaTo))
            {                    
                //Esto no se bien como lo hiciste pero básicamente es la comprobación que quieres
                if (((TipoPpaTo)value).Nombre.Equals("STR") || ((TipoPpaTo)value).Nombre.Equals("TR"))
                {
                    return true;
                }
            }

            return false;
        }

        public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
        {
            throw new NotImplementedException();
        }
    }
}

Then you add namespaceyour converters to the window and create an instance of the newly made converter:

<Window x:Class="MyProject.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:conv="clr-namespace:MyProject.Converters"
    Title="MainWindow" Height="350" Width="525">
    <Window.Resources>
        <conv:TipoToBoolean x:Key="TipoToBoolean"/>
    </Window.Resources>
    <Grid>
        <!-- TODA TU VENTANA -->
    </Grid>
</Window>

Then the TextBoxone you want to disable you control the " IsEnabled" with the SelectedItem of the ViewModel directly, which I think is SelectedTipoPpaTO (or with the SelectedItem of the ComboBox... but if you have the object at hand... better use the object) and send that data to the converter that will return a true or a false depending on the criteria you want to define. I did something like this for the example:

<TextBox Name="TCRM" Grid.Row="11" Grid.Column="1"
Width="100" Height="18" 
HorizontalAlignment="Left" 
VerticalAlignment="Center" 
helpers:TextFilterService.TextBoxFilter="Money" 
FlowDirection="RightToLeft" MaxLength="4" 
Margin="10,2,0,1"
IsEnabled="{Binding ElementName=SelectedTipoPpaTO, Converter={StaticResource TipoToBoolean}}"
Text="{Binding Path=Concepto.TCRM, ValidatesOnNotifyDataErrors=True, ValidatesOnExceptions=True, Mode=TwoWay}"/>

And that's it.. Maybe I misunderstood? Well... I'll leave you the example, tell me if it works for you or I'll modify it.

All the best