Plant linearization for robotic manipulator

Hello Everyone,

I am trying to control the joints of a robotic manipulator and I have used a PID control for the first joint.

While tuning the parameters, I end up with a warning stating that the Plant cannot be linearized.

Why does this happen?

I have attached my file as well.

Also, I cannot control all the joints in a similar manner. Can someone help me out on this?

Regards