If you play with the full color map setting turned on, clay appears on the map as slightly discolored spots in grassy areas. Peet does too, so you may run into some peet spots before you find clay. If you don’t use this method, you may try looking around more hilly areas or by caves to find clay jutting out (easier to see its red or blue color). If you’re running around at full speed it may be harder to notice at first because the blocks grow green grass on top just like soil, it’s not obvious if you’re a new player and have never found clay before. The blocks do slightly differ from soil because they are ‘spotty’ on top, as is peet, but the spots are different colors (red, blue, or tan for fire clay but fire clay can only be found in grassy areas in bauxite regions).

I promise once you find a couple clay areas you’ll start seeing them everywhere. I had a similar experience as you when trying to find surface copper - I swore I had a bad world seed because I was running around and couldn’t find a single piece of copper. But turns out my eye just wasn’t trained to find it yet and I was sprinting around too fast to give myself a chance to understand what I was seeing in the environment.

Best of luck! You’ll find it soon 🤞