Connecting via WiFi to DHCP Server (openhab2, openhabian)

I want to connect to the FritzBox by WiFi. I use a RASPBERRY3.
I have tryed to configure it with “sudo openhabian-config” > "RPi3 WiFi"
If I enter all details SSID & Password it does not work.
The message i get is:

"Not writing to ‘/etc/network/interfaces’, wlan0 entry already available. You might need to check, adopt or remove these lines."
wpa_supplicant: /sbin/wpa_supplicant daemon failed to start
run-parts: /etc/network/if-pre-up.d/wpasupplicant exited with return code 1
Failed to bring up wlan0.
OK (Reboot needed)
[openHABian] Checking for default Raspbian user:passwd combination… FAILED

If I check the status with IWCONFIG. The result is the following:

wlan0 Link encap:Ethernet HWaddr xx:xx:xx:xx:xx:xx
RX packets:27 errors:0 dropped:27 overruns:0 frame:0
TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:12023 (11.7 KiB) TX bytes:0 (0.0 B)

What can I do ?