Nine times in ten this is the network band: Spencer joins 2.4 GHz Wi-Fi only and cannot see a 5 GHz network. It is an easy thing to miss, so start there and work down the list.
Check the band: make sure your router has its 2.4 GHz band enabled. On some routers both bands share one name, in which case Spencer will still find the 2.4 GHz one.
Re-enter the password: type it in again carefully. Special characters are a common cause of a failed join.
Move closer to the router: Spencer's antenna is small, so keep it within a few metres of the router while you set it up.
Restart Spencer: power it off, wait ten seconds, power it back on.
Restart the router: sometimes the router is the one that needs the fresh start.
If none of that works, message us here, or email [email protected], and tell us what happens when you try to connect, and we will take it from there.