Yes, as indicated in the manual: Plane and volume averaged properties can be computed for slabs (2D) only. If you can use instead the SLAB keyword, then it should work. Otherwise, I think post-processing the cube file would be the option, which maybe someone else can give you advice with (I think it can be easily performed with Python, given it follows the standard format, and even not hard to request to a generative-AI solution). If you want to go for the SLAB option, you can also ask here if you get any trouble.
Best,
Marcos