You might have some old Cisco APs like an Aironet 1142 or 1132, and you can't them join to a Cisco WLC, because the manufacturer installed certificates (MIC's) on the APs are expired.
So you have two options.
If your controller's software version is high enough, it will understand the following command, which will allow the APs to join:
config ap cert-expiry-ignore mic enable
if not, change the time on your controller like this:
config time ntp delete 1
config time manual 03/03/18 12:12:12