Your stud finder is not set up to deal with plaster and lathe - it's totally confused and babbling. Try a different stud finder.

Or use adhesive hooks like "Command" hooks

Also, look for a thin molding a few inches below the ceiling - if you have it, it's called "picture molding" and you can hang things from it with "picture molding hooks" and not make any holes in the walls ar all.

Using a hammer, tap on the walls with the same consistency. You should be able to hear the stud locations. When you get close, use a 1/16" inch bit to confirm. The holes you make are easily filled.

The old style magnetic stud finders take more effort but can find the nails in the lath when the electric ones are confused by the density of the plaster and lath.

Use your fist. You will hear the difference.

Pound a thin nail in every 1/2 inch all across the entire room. I did that at our old house and found all the studs - both of em.

To follow up on mxyplx's comment, it is not at all uncommon to find "unusual" stud spacings in really old houses. My own house, the oldest portion of which was built circa 1810, has corner posts and "studs' around the window openings only. This leads to areas close to three and a half feet wide without any vertical framing at all. Since the corner posts are at least a full 6 x 6 inches and the window framing 4 x 4 with 1.5 inch boarding boards on the outside and 1 inch lathing on the interior, it's probably at least as strong as more conventional framing.