Simple script to validate post data before saving it.

block-editor-validation.js

/*
 * Code authored by Valentin Grenier - Studio Val
 * Freelance WordPress Developer from Toulouse, France
 * https://studio-val.fr/
 */

let isNoticeDisplayed = false;
const POST_TYPE = 'your-post-type';

wp.data.subscribe(() => {
	const hasCategory = wp.data.select('core/editor').getEditedPostAttribute('categories');
	const postType = wp.data.select('core/editor').getCurrentPostType();

	// == Check if there are no categories selected
	if (POST_TYPE === postType && (!hasCategory || hasCategory.length === 0)) {
		if (!isNoticeDisplayed) {
			// == Lock post saving and show a notice if not displayed already
			isNoticeDisplayed = true;
			wp.data.dispatch('core/editor').lockPostSaving('require-category');
			wp.data.dispatch('core/notices').createNotice('warning', 'You must select a category to save your post.', {
				id: 'require-category',
				isDismissible: false,
			});
		}
	} else if (isNoticeDisplayed) {
		// == If a category is selected but the notice is still displayed
		isNoticeDisplayed = false;
		wp.data.dispatch('core/editor').unlockPostSaving('require-category');
		wp.data.dispatch('core/notices').removeNotice('require-category');
	}
});