I think it has its own builtin non-rechargable battery; but that should be good if the replacement was a new unit. Double check that all the connections on the holder and on the unit are clean. Also, if it was a new unit it may have come with a sheet of plastic insulting the battery for shipment; things like that often do. Open the battery compartment and check.
Some of its messages are a bit silly when things are not right. For example, if you forget to put the bike battery back in (not the Intuvia one) after charging off the bike, rather than showing 'no battery' the Intuvia shows the state of charge the battery had when last seen. I've pedalled off without the battery more than once. Not helped by being up a hill so it doesn't make much difference at first; by the time I notice I've got the hill to climb back up again without the motor. Luckily not a very long or very steep hill.