For the most part, you should be able to join the Berserkers once you’ve completed enough side quests for the Berserkers. However, if you act harmfully towards their society along the way, Ragnar will become upset with you and deny your entry into the faction and prevent Jax from doing business with them.
If this happens, ask Ragnar for a second chance to prove yourself. This will unlock a quest titled Making Things Right, where you must gain favorability with three specific Berserkers: Cormag, Angrim, and Ragnar himself. Cormag is easy, as he agrees to help you right off the bat. Angrim will agree to help only if you return the favor for him when he goes to become a warrior. As for Ragnar, just pay him 2000 Elexit to get in his good graces.
