The error message indicates that the XMODEL simulator engine could not find the variable driver for an xreal-type signal, which is connected to one or more circuit primitives. This error occurs when there is an xreal-type input port left unconnected, due to some mistakes in the port names, orders, or types while describing a module instance.

Here is an example of such error message:

*** ERROR: [CLM] No connectivity found for the circuit primitive port 'tb.TB1.X3.M2.d'. It might be open or connected to a non-xreal-typed signal.
*** ERROR: [CLM] The following xreal-type ports are not being driven by a proper variable driver:
tb.TB1.X3.M2.d
tb.TB1.X3.M2.g
tb.TB1.X3.M2.s
tb.TB1.X3.M2.b
tb.TB1.X3.a
tb.TB1.X3.y
tb.TB1.X3.vdd
tb.TB1.X3.vss
tb.TB1.a
tb.TB1.y
tb.TB1.vdd
tb.TB1.vss

The first error message says that the XMODEL engine cannot locate the variable driver for the instance port “tb.TB1.X3.M2.d”. However, it is not always straightforward to locate the source of misconnection especially when the port is connected to many other module instances through multiple hierarchical levels. To guide the users finding the source of misconnection, XMODEL outputs the second error message starting from the 2018.12 release. It lists all the problematic instance module ports while walking from “tb.TB1.X3.M2.d” to the top-level hierarchy. In this case, the ports of the top-level module instance “tb.TB1”, namely, a, y, vdd, and vss were left unconnected or connected to non-xreal type signals (e.g. wire) due to mistakes. A good way is to look for other warning/error messages reported by the Verilog simulator regarding the connection of this module.