When a Sigfox transmitter is declared on the Sigfox Backend, its PAC number changes. If you declare a sensor on the Sigfox Backend once and then delete it, you will not be able to declare it again with the same PAC number.
You will have to declare the sensor using the new PAC number that was generated when the transmitter was first installed. The new PAC code can be found in the Device / Deleted Devices section of the Sigfox Backend.