Can you upload your Dorico project, or a cut-down version of it that demonstrates the issue?
Edit: I didn’t see that you had uploaded a file at the top of this thread. @Janus shows that you can change the existing implicit rest without even being in Note Input mode, by selecting the rest; I can also go into Note Input, put the caret at the beginning of that measure, change to the downstem voice, and use the Bars popover to insert the bar rest.
Since there doesn’t appear to be anything in the file preventing the insertion of that bar rest, it seems like it may be user error. Are you in Write mode when you try to use the Bars popover? You can’t make any changes to musical content in Engrave mode.
No, I’d love to know how to do it. I tried making a video with the Windows Snipping tool but it was 9MB and therefore too large to upload here.
Amazingly, this morning I was able to add the whole bar rest successfully while not in Note Input mode. It still doesn’t work when I’m using the caret, so I think it must be something to do with my Note Input settings, although I thought I’d reset them to factory last night.