Checking the following connections and settings might resolve Sensor failover issues:
The Sensor model and Sensor image version on both the peer Sensors should be the same.
The Sensor license and IPv6 status should be identical on the peer Sensors.
Identify the interconnect port for the selected model because the interconnect ports vary for different models.
Check on the FO type setting on the Sensor. The failover creation would fail if the FO type is set on the primary Sensor.
The Sensor health status should be good and normal.