<<no longer works in Dorico 3.
‘thick’ does, : creates the dotted bar line, ‘tick’ creates the tick barline but II … nada.>>
my error - had a temporary aneurysm… || works… II does not. 
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I can’t confirm this. I just wrote stuff in D3 with lots of double bar lines…
On my British keyboard ii or II (Shift+i) do not work, but || does.
|| is shift+\ (backslash).
Maybe you see no bars added, when you select shift-B? Click on the Bars and Barlines icon (9th from the top) in Write mode on the right screen part, and after the text Insert Bars in the Barlines panel, switch from ‚start of flow‘ to ‚end of flow‘, enter the amount of bars you want to enter, and click on insert bars. Does this work?
Sorry, got your post wrong. As being said above, don‘t select the letter I , but the pipe symbol || twice.
Which keyboard layout are you using? If it’s different from English, you have to go to the key commands in General preferences and set your keyboard language again.