If you want global barlines on all staves:
Open the bar popover (shift+B)
Type ‘single’
Press enter
To copy it elsewhere:
Make sure View → Signposts → Time signatures is enabled
Select the red flag
Hold alt(option) and click anywhere in the score.
If you want a barline in one staff only:
Select a note in the staff where you want a barline
Open the meter popover (shift+m)
Type in X
Press Alt+Enter
Select the X
Open the Properties panel (cmd+8 / ctrl+8)
Check ‘Hide time signature’
You can copy it using the same method as above.
What if Alt-Enter does absolutely nothing? Neither key is broken, but when I try this method of adding a barline in a single staff, pressing Alt-Enter does nothing at all.
In order to have independent barlines , you first need to have input an independent time signature on that staff.