Make sure that your layer is in a projected coordinate system (not
lat/long).
In the Processing Toolbox > Vector geometry > Offset Lines
Positive distances will offset lines to the left, and negative distances
will offset to the right of lines.
Setting that I have found useful:
Segments
I am running QGIS 3.16.14
I have a shape file layer with a multi segmented line and would like to snap a
parallel line on one side at a fixed distance. I tried the parallel line plugin
but it wouldn't make another line in any visible layer. There was an error that
showed briefly that my map was
