Vertex Weight Issues

So, I’m trying to import a character model into another engine that only holds support for 3 weights per vertice, now I know you’ll say simply use Limit Weights, but there in lies the issue:

Whenever I try this with any model, I get the following

I have no idea what’s caused it to be greyed out, nor have I ever been able to fix it.
I really have no ideas where to go from here. Any help would be appreciated