I'm having trouble setting up an FEM slide constraint. I get different errors in the DOP network depending on which node I have selected and if I've run a simulation or not. After running, the FEM solver has the warning "Failed to read state for slide constraints".

As a test I tried to replicate the video found on this documentation page https://www.sidefx.com/docs/houdini/nodes/dop/femslideconstraint.html [www.sidefx.com]

I've attached the file and a screenshot of the DOPs setup.

I've struggled to find much information about FEM in general. Looking through the documentation, and using a few tutorials on the sideFX website, I've been able to piece some things together but I get stuck frequently. I'm not sure if its just a lack of information about how to use FEM, or my lack of experience with DOPs and Houdini in general. Any tips on how to get a better grasp on FEM are greatly appreciated!

The error you're getting here is a bit deceiving.
The actual problem is that the ground plane is being run through the FEM solver, which doesn't know how to solve ground planes.
As a result, the solve fails, and as a side effect of the solve failing, the error that you're seeing shows up.

To quickly fix this network, delete the ground plane and add a new ground plane using the Ground Plane shelf tool in the Collisions tab.

The quickest way to learn more about FEM is to use the shelf tools and look into the DOP setups that they create.
It's also worth looking at the user guide for "finite elements" in our help docs.
Most aspects of an FEM simulation can be controlled by means of SOP attributes, which are discussed in the help of the FEM Solver node.

I hope this helps

This helps a lot. Thanks!