This image cannot be processed by the web server. Convert it to JPEG or PNG before uploading

From your comment I understand you are trying to upload a webp image. This format is supported since WordPress 5.8. However, for this to actually work, your server needs webp support as well. The error message says this is not the case.

More in detail: the PHP installation on the server comes with one or two image libraries that do the hard work in handling media: ImageMagick and LibGD. Both in principle support webp, but not in all installations. It looks like on your server, they do not.

Since WP uses ImageMagick by default, you could try switching to GD to see if that library has webp support on your server.

add_filter( 'wp_image_editors', 'wpse425693_prefer_gd_over_imagick' );
function wpse425693_prefer_gd_over_imagick($array) {
    return array( 'WP_Image_Editor_GD', 'WP_Image_Editor_Imagick' );
}

If this doesn’t work you’ll have to contact your hosting provider and ask them to add webp support.

UPDATE

For those who wonder why providers may be reluctant to allow webp support: there are security issues with this format. Google for “webp vulnerability” to get the latest news.

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