Xcode 13 was required

@nbe I never got notified you replied. I am able to set, get and remove session and local storage from my background page/script without issue. I am not using the safari converter however, so perhaps it's an issue with that or how you are going about setting/getting. That being said the person who replied has good advice, I'd do what he says.

Where are you trying to store session data? You mentioned your background script, but it is unclear if you are trying to set session data for the background page, popup, extension page or on a webpage via content script.