ARC Trooper Turret

Maybe it takes up the ally slot of which you can only have one? Try going in with no ally selected. (If that’s even an option? It’s been so long I don’t remember.)

Sorry, I missed that you’re having an issue in arena and raids too. No idea.

The turret doesn't appear in raids for sure.

The tables it won't show if you have an ally chosen as Liath suggested.

No clue in arena unless you are bringing in GBA with the presummon zeta.

Not sure about the others, but turret can’t be summoned in raids at least.

If you are battling in say darkside you get to use an allies toon for that battle. Because of that ARC troopers turret does not show up. If you want the turret for that battle instead of an allies toon before you start the battle click on the allies toon and remove it. You should get the turret then. Also the turret will not show up in the raids. I don’t know why they won’t let us use it probably to difficult to add in for the raids.

Weird that it doesn’t show up in arena, but it can’t be summoned in raids.

I'd like to see a screenshot of arena. I suspect OP is misremembering (or using GBA as I suggested).

Try using it in GW just to test.