What you may want to look at in Polish is "Muzyka na żywo" Szczecin. I'm not from there so can only give you a link to some clubs with live music:
kluby.cjg.gazeta.pl/klub/miasto-Szczecin/muzykanazywo-Tak/1
Also check the Free Blues club - they sometimes organize jam session nights.
facebook.com/FreeBluesClub
and maybe Stara Komenda Browar:
facebook.com/Stara-Komenda-Browar-123716704358258/