Help with WPF style

I'm building a WPF desktop app. I've been following this tutorial https://youtu.be/PzP8mw7JUzI?t=991 to figure out how to make a reusable style for a button that has two text blocks (one for a fontawesome icon and one for the label). Sometimes the preview shows the style being correctly applied, but when I build and run the app, it shows that "error" button with the message The element [Button] could not be displayed because of a problem with System.Windows.Controls.Button. and the app doesn't open. [more context in next messages]