Image size is smaller than recommended AMP ???

We checked your issue. This is actually you have got the error is because of the Yoast SEO plugin. Please follow this tutorial.
Reference: https://ampforwp.com/tutorials/article/how-to-solve-image-size-smaller-than-recommended-size-in-amp/

Is there any other way to fix it using code ?, use the “Schema & Structured Data for WP & AMP” plugin but I delete all the json-shema code perzonalized from the web, both in desktop and mobile view (Amp)

Here there is no need to add anything in code. You need to set up structured data properly.
WordPress Dashboard -> Structure Data -> Settings -> Compatibility
And checked the box of Yoast in the Compatibility section of the options panel and save it.