This issue is also on 2b2t, most people say that it is from an anti-elytra-hack plugin.
Also, it's spelled elytra, not elytro.
I don't think so.
Do you have Microsoft Family? If so, ask your parent to follow these steps to unblock multiplayer:
Go to https://account.microsoft.com/family/
then pick More Options > Xbox settings - this will take you to the Xbox account, that is linked to the MS one and there go to Privacy & online settings, then pick him and the second tab is Xbox1 / Win10 online security page where you can allow the kid to play multiplayer games (second option from the bottom).
Note: I got this from this page: Minecraft Java Edition - Multiplayer Option disabled - Microsoft Community so I apologize for the confusing instructions.
If you do not have Microsoft Family or multiplayer is only blocked for Optifine, then you might have to reinstall Optifine altogether. If THAT doesn't work either, then reinstall all of Minecraft, and if you're STILL having issues, then idk.
In adventure mode, you can't break or place blocks. Go to survival.