How to implement if, else statement in CanExecute command or Delegate command in wpf c#?

new to WPF and prism. How do I use an if else statement when using CommandCanExecute(), CommandExecute()/Delegate commands?

I have a code to load and get the instance of livechart. However, if the file for the chart does not exist in the users desktop, my application will crash. I want to implement a if else statement to say if you cannot find the graph, show up a message box notifying that there is an error, instead of crashing the program.

I tried searching up on RaiseCanExecuteChanged but unsure how to implement.

   private bool BEYieldCommandCanExecute()
    {
        return true;
    }

    private void BEYieldCommandExecute() 

    {
        if (true)
        {
            _eventAggregator.GetEvent<GraphPubSubEvent>().Publish(_viewName);

        }

        else
        {//Check
            MessageBox.Show("Error loading. Please ensure Excel file/Server file exist in Desktop/Server to generate Chart.", "Invalid Request", MessageBoxButton.OK, MessageBoxImage.Exclamation);
        }
    }

Thanks alot!!