I have all 4 of the prerequisites for it
-Physical projectile damage 3
-Military tactics
-Military 3
-Portable solar panel

But it's telling me that I don't have one of the prerequisites. Is it some hidden tech I need? Why can't I research it?

Hello,
you may have a mod that modifies existing research present in Factorio.

The "Sniper rifle" requires 4 technologies
- Military 3
- Military tactics
- Physical projectile damage 3
- Nightvision equipment

A snippet from the code:

local sniper_rifle = {
  name = "sniper_rifle",
  prerequisites = {"military-3","advanced_military_tactics_1","physical-projectile-damage-3","night-vision-equipment"},
...

As you see, there is no need for "Portable solar panel", but instead for "Nightvision equipment", and as of now even the crafting recipe depends it.
If it really does require something else, please share details, so I can fix it or mark the mod as "conflicting".

Ah, then the nightvision overhaul mod, which adds more tiers of night vision seems to break it.

You mean the
"Nightvision Overhaul" by "NOiZE" right?
I can see that their mod hides the vanilla recipies. Thus making you unable to actually research the "Sniper rifle".
I'll see what can I do about it.

In the meantime you can do this
(cheats)

/c game.player.force.technologies['sniper_rifle'].researched = true

or you can alternatively do

/editor

and research it manually with that.

As a last option you can also disable the "Nightvision Overhaul" mod, research "Sniper rifle" and then reenable it, but this comes with a cost of you needing to research the NVG tech once again and same with the items.

Hope this helps.

Thanks!

Fixed in 1.3.1
It is now an optional dependency