ProMods

I want to connect the road to the prefab (green node to green node), but it doesn´t work. Do you know why?

First of all, you need to change the origin (red dot) on the prefab! Looks like you have set it to 1, and that will not work when you are going to connect the two other roads (because two red dots can't connect).
For a prefab like that, I would set the origin to 0. Then it will be a red/green dot connection. If you then have trouble with connecting them, try hold the 'Alt' key when connecting. That should do it.

Thanks, will try it.
Edit: Still doesn´t work, even if I hold ALT

Are the road and prefab nodes at the same height?

I would say so.

(N) Node Properties and set same height to this nodes.

I checked it, all Nodes were at the height 0.00, but it still didn´t work, also while pressing ALT.

Did you change something here?

It looks exactly the same like in your pic.

Already tried the other way round: to connect the road to the prefab with pressing there alt as well?