To display a product attribute as a filter on the category page, the attribute must meet the following requirements:
1. Click on 'Stores' and then click on 'Products' (which is found under the 'Attributes' section).
2. Select or search for the desired attirbute and then click on it.
Once the attribute opens, you can see all the 'Attribute Information' tabs.
3. Click on the 'Properties' tab (the 1st tab), proceed to the 'Catalog Input Type for Store Owner' field and then select either the 'Dropdown' or 'Visual Swatch' type.
4. Click on the 'Storefront Properties' tab (the 3rd tab) and proceed to 'Used in Layered Navigation' field then select 'Filterable (with results)' or 'Filterable (with no results)'.
Note: To make sure your changes are saved, remember click on the 'Save Attribute' button.
5. If the filter is not visible on the Category page, go to 'Catalog' and click on 'Categories'.
Lastly, select the category, which has the attribute that you wish to display, and proceed to 'Display Settings' section.
Make sure that the 'Anchor' field is set to Yes.
Note: To make sure your changes are saved, remember to click on the 'Save' button.