Item without tax

How can I register a product only that does not apply any tax in a purchase or sales order?

I don’t think an option like this exists, seems like you are asking for doctype + item combination to determine tax. Currently you can remove the defaulted taxes from your transaction if they are not needed.

In Peru there are products that do not apply taxes. Example: Medication. A purchase order has 3 products and one does not apply tax. In the tax grid there is no way to separate only the product that does not apply tax

In that case, at Item Tax level you should not have tax set up for that Item. It would default accordingly. It is important to understand the combination (transaction + item or only item) which determines the tax.

I think you can define zero tax in item, is it will work with you