vegacorp

Jose Vega

Force users to select a product category when creating or editing WooCommerce Products. For example, make category required.
Prevent users from creating products without category.
You can change the validation message in the settings page, by default it says: “Please select {taxonomy_name}”. Example: “Please select categories”.
<h4>Use cases</h4>
<ul>
<li>Force users to select a category when creating products</li>
<li>Force users to select a category when editing products</li>
</ul>
<h4>Go Premium</h4>
<ul>
<li>
Force users to select a tag when publishing WooCommerce Products
</li>
<li>
Force users to select Sizes (Product Attribute) when publishing WooCommerce Products
</li>
<li>
Force users to select Color (Product Attribute) when publishing WooCommerce Products
</li>
<li>
Force users to select Global Product Attributes when publishing WooCommerce Products
</li>
<li>
Force users to select a category or tag when publishing Events
</li>
<li>
Force users to select a category or tag when publishing blog posts
</li>
<li>
Force users to select any Taxonomy Term when publishing any custom post type
</li>
</ul>
<a href="https://wpsheeteditor.com/upgrade-required-taxonomies-wporg" rel="nofollow ugc">Buy Premium Plugin</a> 
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Require Categories when Creating Products – WooCommerce

I can use it to force my clients to select category when creating products.

Great

Force users to select a product category when creating or editing WooCommerce Products. For example, make category required.

Denis Yanchevskiy

Allows you to make post, page, custom post elements required: title, content, featured image.
After installing the plugin, you need to go to the Settings -> DCO Post Validator to specify the required elements.

DCO Post Validator

Allows you to make post, page, custom post elements required: title, content, featured image.

Require Categories when Creating Products – WooCommerce vs DCO Post Validator