Can ChatGPT design an ad poster?
It can write the poster's text and describe a visual precisely enough for an image model. Expect to fix letters and numbers in the result, and to iterate on the framing. For a printed poster you still need a proper layout. The rest is layout.
Prompts that actually work
Describe the subject, the material, the colours, the lighting and the empty space you need for the headline. Ask explicitly for an image with no text on it, otherwise the model will try to write something and invent words that were never in your brief. Name the format you want, because vertical and square behave differently.
Why the text breaks
Image models approximate letters rather than set them. Anything longer than a short word usually comes out distorted, so keep the image clean and add the type afterwards in an editor where you control the size, the spacing and the spelling of every line. Keep a copy of the clean image so you can rewrite the text later.
Prompt or dedicated tool
Prompting gives you full control at the price of iterations. A generator that starts from your website removes the prompt work but imposes its own visual style, and it makes vertical digital ads rather than print-ready posters. Choose according to the format you actually need. Neither is wrong; they answer different questions about control and speed.
Examples made with Adrefyn