Try buffer-face-set, which accepts either a face name like (buffer-face-set "error") or a property list of face attributes and values like (buffer-face-set :weight 'bold :height 150).
I think your specific example would be something like (buffer-face-set :font-family "American Typewriter" :height 150).
You can M-: and at the "Eval:" prompt enter one of ...
With respect to what you tried using set-face-attribute:
The error message tells you that the first argument needs to be a symbol. At that point you should use C-h f set-face-attribute, which tells you that the first argument needs to be a face.
Here's an example of its use.
(set-face-attribute 'default nil ':font "American Typewriter" ':height 150)