My issue was answered in the support channel within seconds. The RPGSessions discord is very helpful and friendly. For others experiencing a similar issue. The GM also needs their own"player character" in the list of PCs, even if they aren't going to use it. Then you can select and roll initiative with your adversaries as normal.

I wonder if I am doing something incorrectly with NPCs in the game table. I am unable to roll NPC initiatives when I have them added on the right-hand column under adversaries. I get an "internal server error" when I click Roll NPC.

I heard this was amazing via the Forge podcast so time to dive in.