I have a Fitbit Aria scale with wifi. Before I "upgraded" my router, it was on my home wifi network. Now it isn't. I have followed the Fitbit instructions to connect. I think it's trying to join the network, but the network doesn't "see" it.
Check and see if you have a 'dual-band' router. If the router is capable of broadcasting both 2.4GHz and 5GHz wireless signal, but you only see one wifi network name...that's where the problem is. Your fitbit can't connect to it because it needs the 2.4GHz wireless band...but because it is combined with the 5GHz network, it doesn't accept it.