Failed to deploy a selected AP

No not using virtual domains .. I just register this device to my account , then I need to change its Wireless SSID and password , so I go to the fortilan and tried to deploy it to network field and massage above appearing "device is already added in a different domain , please contact Fortinet support " . 

Has the device been registered before with another account/customer? I think you should open a CS ticket.

You need to open CS ticket and request to move the device to the new account. In case it is eligible then they will send a mail to the initial owner for validation.