Hello Simay,
As per my understanding, while converting a Simscape Model to RigidBodyTree you are getting the mentioned error.
URDF (Unified Robot Description Format) do not support a closed chain based structure. Hence, while converting your model from Simscape to RigidBodyTree you are getting an error to remove the closed chain structures.
As the error suggests, to successfully convert a Simscape model to RigidBodyTree, you can set the "BreakChains" Name-Value pair to "remove-joints" while using the "importrobot" function. The "remove-joints" will automatically remove the chain closure joints in the final output. An example code for the same would look like:
importrobot(model, "BreakChains", "remove-joints");
For more information, you can refer to the following documentations:
