In my case this problem appeared when I had already a wifi set and wanted to change it for another one.
I solved it from the terminal:
LibreELEC:/etc # connmanctl
connmanctl> agent on
Agent registered
connmanctl> scan wifi
Scan completed for wifi
connmanctl> services
WIFI_SSID wifi_XXX_managed_psk
connmanctl> connect wifi_XXX_managed_psk
Agent RequestInput wifi_XXX_managed_psk
Passphrase = [ Type=psk, Requirement=mandatory, Alternates=[ WPS ] ]
WPS = [ Type=wpspin, Requirement=alternate ]
Entered the passphrase and it worked.
The problem was the agent on, apparently it was off...