Your ap is searching for a controller which is expected behaviour for a lightweight access point. For your purposes i beleive you will need to convert the ap over to ciscos mobility express, to allow it to operate as what used to be refered to as autonomous.
I’ve not played with mobility express personally, but you should be able to find info on the googles to get it sorted, else im sure others on here have given it a crack before.
I found " AIR-AP3800-K9-ME-8-10-151-0.tar " and " AIR-AP3800-K9-ME-8-10-151-0.zip "
I know that I have to do something with Tftp but don’t know how to.
Find a TFTP server application and add those files to it. You should then be able to have the AP access them via TFTP. There are several free ones out there.