When creating a collection on Shopify, there are two possible types of categories that you could create: An Automated collection, and a Manual collection.

At the moment unfortunately, Modalyst does not support the created of automated collections, which may be the reason you don't see your collection when attempting to categorize a product.

We only support manual collections. To learn how to create a manual collection, check out this article on Shopify: https://help.shopify.com/manual/products/collections/manual-shopify-collection