iPhone won't erase?

I'm just trying to restore to iOS 12 from the current beta iOS 13 I have installed because my Apple Watch wouldn't pair and so apple support suggested (after we tried everything else ) to erase all content and settings and go back to iOS12 - which fine...didnt want to do that but it seems the only choice if I don't want to wait for iOS13 to launch officially

So I try to do that and backup the phone and hit erase all content - enter Apple ID - did that - and then it says "Cannot Erase iPhone" There was a problem removing this iPhone from your account. Try erasing this phone again"

My brother told me I had to turn off Find My iPhone so I did that and then it says "Cannot Turn Off Find My iPhone - There was a problem removing this iPhone from your account. Try turning Find My iPhone off again"

I'm on an iPhone X Max and I have the beta iOS13 installed as I said so just don't know what to do at this point so and advice would be appreciated

JF