Hi,
Based on your description, I think you created a test harness for a subsystem in your design model, and setup Simulation option 'SIL' for Simulink test using Test Manager.
In test harness, SIL mode is not supported. The reason is that we can create a SIL harness for a subsystem in the design model. During creating the test harness for a subsystem, you can specify the verification mode as "SIL" to create a SIL harness. Then you can use normal simulation mode to simulate the test harness.
Please see the following web page for the detailed explanation about creating a SIL harness for a subsystem. https://www.mathworks.com/help/sltest/ug/silpil-verification-for-a-subsystem.html .
From bulletin 8 and 9, those are the steps to create a SIL harness.
Hope it helps !
