Error on combine test or trial functions from different spaces

[dokken]

I’ve already told you that you should use a Mixed-element.

@dokken

General comment to your code. If you want to solve your problem for both V and W, you need to use a MixedElement. See:
https://docs.fenicsproject.org/dolfinx/v0.6.0/python/demos/demo_stokes.html#non-blocked-direct-solver
or any of the approaches further up if you want a more technical (But better perfoming) code.

In general, I would strongly advice you to familiarize yourself with the demos
https://docs.fenicsproject.org/dolfinx/v0.6.0/python/demos.html#demos
and if you want more details, consider the tutorial:
https://jsdokken.com/dolfinx-tutorial/