Custom Attributes don’t stick, and PHP warnings after import

I’m trying to add custom attributes to variable products, for example “color” & “size”. (Using VillaTheme Dropship & Fulfill for all imports)

If the product only has “color” before I import it, and I try to add a “size” attribute with a few variations, it all seems to work fine until I click on “Save attributes”, then all the variations and the size attribute itself disappears.

If the product has attributes for “color” and “size” before I import it, then after the import the color variations work fine, but if I expand the size attribute, I get PHP warnings instead of a list of the imported size options. Here is a sample of the warnings:

Warning: Use of undefined constant VIWPVS_WOOCOMMERCE_PRODUCT_VARIATIONS_SWATCHES_TEMPLATES – assumed ‘VIWPVS_WOOCOMMERCE_PRODUCT_VARIATIONS_SWATCHES_TEMPLATES’ (this will throw an Error in a future version of PHP) in /public_html/wp-content/plugins/product-variations-swatches-for-woocommerce/includes/templates/html-product-attribute.php on line 170

Warning: include(VIWPVS_WOOCOMMERCE_PRODUCT_VARIATIONS_SWATCHES_TEMPLATEShtml-global-attribute-item.php): failed to open stream: No such file or directory in /public_html/wp-content/plugins/woocommerce/includes/wc-core-functions.php on line 345

Warning: include(): Failed opening ‘VIWPVS_WOOCOMMERCE_PRODUCT_VARIATIONS_SWATCHES_TEMPLATEShtml-global-attribute-item.php’ for inclusion (include_path=’.:/usr/local/php73/pear’) in /public_html/wp-content/plugins/woocommerce/includes/wc-core-functions.php on line 345

Warning: Use of undefined constant VIWPVS_WOOCOMMERCE_PRODUCT_VARIATIONS_SWATCHES_TEMPLATES – assumed ‘VIWPVS_WOOCOMMERCE_PRODUCT_VARIATIONS_SWATCHES_TEMPLATES’ (this will throw an Error in a future version of PHP) in /public_html/wp-content/plugins/product-variations-swatches-for-woocommerce/includes/templates/html-product-attribute.php on line 170