Export button hidden under task bar

What is your scaling % set to? https://helpx.adobe.com/photoshop/kb/hidpi-retina.html 

This is usually an issue with screen resolution not being high enough.

However, I've noticed that in the latest version of Photoshop this dialogue box is massive (I'm on Surface Pro with 2K screen and it doesn't fit, either....) so it looks like you need to resize it from the top the way I have been doing it ever since the new version came out...  😞