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Did you disable the self-protection feature in settings> Troubleshooting 
Yep. That works! Thanks! So either there is a bug in Avast or an omission in the instructions. Easiest fix would be to update the latter. Users shouldn't have to resort to the forum when the failure is due to incomplete documentation.
This is an acknowledged bug - work around is the option given - this bug I believe has been squashed in a previous beta version
https://forum.avast.com/index.php?topic=186192.0 11.2.2263.
+ Fixed creation of Rescue Disc
So it shouldn't be too long before this is resolved without having to disable the self-protection function.
