You're using PhotoStack Classic, a variant of PhotoStack intended for use with legacy web browsers and platforms, including Internet Explorer (10+), Edge on Windows 10 Mobile, and Safari on old iOS devices. Some features in the primary PhotoStack web app are unavailable in the Classic app.
Watermark files created with newer versions of PhotoStack can be imported in Classic, and vice-versa.
You can access the main PhotoStack application from edit.photostack.app.