Unable to add managed FortiAP - Invalid serial number

Hello folks

I'm trying to add a FortiAP-221E in the managed FortiAP section of my FortiGate 100E and I keep on getting invalid serial number. I actually type in the serial number written on the back of my FortiAP. I configured the port on the Fortigate which the FortiAP is plugged into to be Dedicated to FortiSwitch as stated into the documentation. The FortiAP doesn't appear in the managed AP list of my FortiGate. Thats why I'm trying to add it manually.

Can anyone help?