Using Amecs API 1.1.5+21w16a on Minecraft 1.17.1.

Crash report: https://pastebin.com/vEB7cFnf

Well, you're using an outdated version of NMUK :)

Try updating that to 1.0.1+m1.17.1

I don't have a dependency on NMUK (I guess Amecs API includes it), but okay I guess?

oh, true.
Amecs API requires this for testing purposes. Sadly afaik I can't turn that dependency propagation of NMUK off.

So the way to go would probably to exclude NMUK in your build-script:

dependencies {
  modImplementation("de.siphalor:amecs-1.17:1.1.5+") {
    exclude module: "nmuk-1.17"
    // or
    transitive false
  }
}

That's about what I did, yeah. Thanks for the help!