There is a critical bug occurring when creating curves in projects with URP in 18.5.555-18.5.568. This is fixed in Houdini 18.5.569.
Alternatively, you can fix this yourself in Assets/Plugins/HoudiniEngineUnity/Shaders/URP/Color.shadergraph and change the path from "Houdini/UDP" to "Houdini/URP": https://github.com/sideeffects/HoudiniEngineForUnity/commit/25401c0eb5a965fb6aa81dc9901fcd43683d1bc9 [github.com]
The error looks like this:
EDIT: It appears that this may leave the plugin in an unstable state. It may also cause an issue where many curve objects are created, as mentioned in https://www.sidefx.com/forum/topic/79152/ [www.sidefx.com]