Can I change the clock face from my iPhone

Yes, you can. Go to the watch app on the iPhone, then in the bar below to watch faces (the middle option) here you can select a new watch face and install that one.

If this is the first time that you're changing a watch face, you can only do it via the iPhone as you first need to download the new watch face, before you can select it while holding the screen and swiping right

I can't seem to change my clock face on my Apple Watch

I want to change my clock face on my I watch and it won't let me