Can't pick whip or parent to specific layers (> layers are greyed out in dropdown)

Hi there!

It's possible my question is a little hard to answer (or the answer is really simple).

Basically, I want to parent one layer to another one, but I can't. The pick whip won't stick and in the dropdown the layer I want to parent to is greyed out (others are not and I can connect to them, no problem).

Maybe you already know what is wrong?

If not, I might have to go deeper:

I'm trying to rig a character using the Duik Bassel plugin. This plugin creates a rig that I can parent my illustration to. It also creates controllers that are tied to the rig in order to animate movement. All these layers are shape layers set as guide layers.

When I now build my own rig the plugin connects some of the structures not like in its demo rig, resulting in weird behaviour of the rig.

When I try to connect the plugin layers like its demo has them connected, this is acutally were my problem emerges: I can't connect them to the right layers because they are greyed out.

Now, I know this is a non-Adobe plugin, but maybe someone still knows an answer, as the problem might not even be plugin-related...?

Anyways, thank you for bearing with me 🙂