For UR eSeries controller:
Make sure that the UR is connected. Switch from "Remote Mode" to "local Mode" and back again.
Alternatively and for UR CB3 controller:
Another option is to re-establish the connection to the MIRAI controller. To do this, press the "connect" softkey in the Polyscope under Installation and URCaps in the selection area.
Also check if the drives units of your UR are activated.
If the error still occurs, please contact customer support.