Detailed error as follows:
https://pastebin.com/ik4UCYSi

Server Details:

• 1.21.4
• Fabric Installation w/ Fabric API

Extra:

• Installed Fabric API and Forge Config API Port as instructed on the modrinth page
• Modrinth says this mod is Server & Client compatible. Have not tested on a local world.
• Issue based on a Critical Injection Failure
  • The rotate method does not exist in net.minecraft.class_2482 in the current version of Minecraft (1.21.4).
  • The method signature has changed, been removed, or is misidentified due to incorrect mappings.

Reproduction:

• Run a Fabric 1.21.4 server
• Install Fabric API, Forge Config API Port, AdditionalPlacements
• Start the server

What Happens:

• Server crashes when AdditionalPlacements is initialized

What Should Happen:

• Server loads successfully

We cannot currently check if this is a mod incompatibility. Based on the information provided, we hope you find this helpful!
If any more information is required to aid in fixing this issue, just ask.

Can confirm this is also happening for me on Minecraft version 1.20.4 using Fabric.
Logs in pastebin:
https://pastebin.com/JT3EjpMc

It's been a few years since I touched fabric mod making, so I'm pretty rusty. But could this be a mapping issue?
Logs suggest to me that mixin Injections are missing their targets. Log snippet for reference:
Critical injection failure: @Inject annotation on getStateForPlacement could not find any targets matching '@Desc(owner = class_2440.class, value = "getStateForPlacement", args = { class_1750.class }, ret = class_2680.class)'

Hope this helps 😃

Fixed as of the latest commit and release