Hi Jojo Tra My,
Since the Xbox privacy settings are already set to Allow and restarting the PC did not change the Multiplayer option, you can try refreshing the Family Safety configuration on Windows.
- Press Windows + R, type
cmdand select OK. - Run:
schtasks /run /tn "Microsoft\Windows\Shell\FamilySafetyRefreshTask" - Restart the PC and test Minecraft again.
If possible, also verify that Minecraft Launcher, Xbox app, and Microsoft Store are using the same Microsoft account.
I hope the refresh clears the remaining restriction. If Multiplayer is still unavailable afterward, that would help narrow the issue further toward the account/family configuration rather than the local Windows settings.