Just starting out with Houdini 15 Apprentice and running into odd problem: when I lay down a node (Polyextrude, for example) the node has red lines in it. I looked this up and it indicates that the node is not cooking. No matter what I do, the node never cooks. However, when I close the program and restart the scene, the node will cook properly and the red lines are gone.

I've attached a photo. I'm running H15 on a Windows7 64bit computer with Titan X GPU and 16Gb of ram.

Any thoughts on how to fix this problem?

It errors because it´s not connected.
The reason it doesnt error after restart is that your selection(yellow border) is “neutralized” on startup.

Plug the output of polyextrude1 into the top left input on polyextrude2.

Middle click a node to get info about errors and warnings and on the inputs to get info about what is expected to be pluged there.

Have fun!

By golly, that works! Thanks for clearing that up for me, mawi.