Hi Jesus! Product types cannot be outright deleted. It would 'orphan' products and leave them with no product type (not allowed) You would need to change the product types of the products in question. When there are no more products with that product type it will simply go away.
This is a big problem when dispatching product upload tasks to others and they are inputting tags and product types. When a tag or product type, that was for example a typo, is mistakenly chosen, the product does not auto populate to its store collection and is not showing live. There needs to be a "clear text" only way to delete the text in: tags, product type and collections without it affecting the products already uploaded with those keywords in those fields. At this point, Shopify just told me it is not possible but they are sending a ticket to look into this. Well the first ticket on this was in 2013 and this is 2020. It's a big problem for stores with a lot of products.