Trying to use the
OpenFileDialog is really a very common mistake that UWP newbies commit especially those coming from .NET framework.
I can't deny that UWP is a .NET technology that targets .NET core itself. But it doesn't target .Net Framework in its entirety.
OpenFileDialog is part of
System.Windows.Forms and it's not part of .Net Core libraries. You should use
FileOpenPicker instead for Windows Store Apps.