Unknown sources appears if the developer mode is enabled. So first check that on your headset and now the unknown source sections is under a folder icon with a "?" symbol
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Like I said, I have turned Developer Mode on and off multiple times. Unknown Sources shows under my wife's acct when I log into hers but not mine. Mine is the primary/admin acct and the acct I was logged into when I first did all of this. I have used different USB-C cords, restarted the Quest, toggled Developer Mode on/off multiple times, reinstalled the ADB USB drivers, and even installed the Meta Developer Hub. The only thing I have not done yet is delete my wife's acct from the Quest or reset to stock. Sidequest connects fine and is even sideloading QGO because when I go in my wife's acct and look under Unknown Sources, it shows the .apk file with the current date so I know it's installing the apk just fine, I just can't see Unknown Sources on my acct so I can't install QGO.
I don't know how to help you... On my Q3S, I have 2 accounts. First have the new icon, but the second have the unknown sources in the list... So weird too but the fact is I have access to unknown sources on both of my accounts... You should try to google that or to do a factory reset. There seems to be a Meta bug on your headset...
Anyway, if you find a solution, please tell me, I'm curious