I use this one. Although made for baby poop stains, it works for all stains! https://chemistrycachet.com/homemade-baby-poop-stain-remover/
use this homemade oxiclean recipe: https://www.craftylittlegnome.com/homemade-oxiclean-recipe/
All with Oxiclean work as long as you pretreat the armpits with same.
Hello Ignazio, these may help you, each is a different method, so hoping at least one works for you
Oxi-Clean will knock out those pit stains! Mix Oxi-clean with water in a small basin and add your pit-stained article to it. Let soak for a 1/2 hour then launder as usual.