You can't just slap a PAX mod onto any weapon. The weapon has to have a specific slot in order to fit it. Scope and Sight slots cannot be fitted with PAX.
And as I've mentioned in earlier posts, Tier 1 and 2 weapons do not have slots that accommodate the PAX mod. Plus stores usually only carry one PAX mod and do not restock them, forcing players to be extra picky on whether they should apply it to the first weapon that allows for a PAX mod, or save it so they can continue upgrading it to it's top tier rating and apply it to a higher level weapon. Once you apply a PAX mod you cannot remove it.
You can slap it onto any non-Iconic item. Mod slots are T2+ or T3, so pre-10, you may have to pistol whip or shoot them in the kneecaps or use hacks, or beat them with a bat. Bat is my personal choice. Wilson at your main apartment building will restock them every single day and even at level 1 (Well, you're 2 by then, but still), you can buy a white one for 420 eddies. Can't use it yet, which is weird tbh, but it does exist and it is usable and it is super duper consistent. There's only a few vendors that will sell PAX, but Wilson at your apartment is one of them and he, in my experience, always has one. I even loaded a fresh save with 20 minutes in it, bought one off Wilson just to reconfirm it, then skipped a day and another sat in its place. Went back to the save, bought it again, skipped a day, was there again. He sells it consistently.