The Keysight B2961A/B2962A shows the message "Interlock terminal is open" and don't accept to input voltage more than 42 V.

To prevent shock hazard, the Keysight B2961A/B2962A will not operate above 42 V unless you connect the interlock terminal. The terminal should be connected to a switch in the fixture lid or prober door. When the switch is closed, the Keysight B2961A/B2962A works with voltage up to 210 V.
Please refer to the Users Guide of the B2901/02/11/12A in detail.