Syitching via's are used for tying same signal on multiple layeers ( VDD or GND).
Are you trying to add GND on signal layers to fill open areas?
If so select the net and RMB select ADD VIA, start placing them with mouse click.
Another way to add stitching via's is to use the Via Patterns feature under Tools/Options/Via Patterns Tab.
Select a net (GND or VDD) and make sure it appears in the Nets/Via Type Box.
Select the via type that you want to use - it defaults to STANDARDVIA.
Specify the via spacing (defaults to 2.54mm/100mil/0.1inch).
If you want to fill the board with via's, select Fill, then select either aligned or staggered. If you want to fill the outline (perimeter only), select this option.
Once this is set up, exit out of there, and flood your board.
Next, select a segment on the copper flood, RMB, Select Shape. RMB again, Via stitch. It will give a warning, click yes.
Presto, board stitched.
Watch out, however. This process is NOT intuitive, and it can create shorts. Check your layout very carefully, and move/remove any via's that are problematic.
The Via Stitch command may place vias that cause clearance violations if DRP is not turned on. In recent versions Layout will notify you before doing the Via Stitch that DRP is off - do you want to continue. Say no, then go to Options and turn DRP on. This will prevent any clearance errors from being created.
Please note that the Copper Pour or Plane must be flooded for the Via Stitch command to work. It will not work on unflooded planes and pours.