Try to add AP to SSID: There is an Access point that does not support this SSIDs frequency.

Check the AP itself. Likely it claims, not enough power. 

Then follow up with this: https://support.sophos.com/support/s/article/KB-000045707?language=en_US

ok, AP's are prepared, but offline currently.
I asked the customer to try to connect one.

device-log:

Jan 1 00:01:20 localhost local1.info systemd: [S1]: [admin]: Warning: AP is powered on insufficient power (12.95W) operating in AF mode.
Jan 1 00:01:20 localhost local1.info systemd: [S1]: [admin]: (12.95W) 1. All radios disabled 2. Alert on web page for insufficient power
Jan 1 00:01:20 localhost local1.info systemd: [S1]: [admin]: Warning: AP is powered on insufficient power: 12.95W

device-log:

Jan 1 00:01:20 localhost local1.info systemd: [S1]: [admin]: Warning: AP is powered on insufficient power (12.95W) operating in AF mode.
Jan 1 00:01:20 localhost local1.info systemd: [S1]: [admin]: (12.95W) 1. All radios disabled 2. Alert on web page for insufficient power
Jan 1 00:01:20 localhost local1.info systemd: [S1]: [admin]: Warning: AP is powered on insufficient power: 12.95W

Did you check the KB above? There could be LLDP in place or something reports the AF Mode to the Access Point. 

The network team reviews the requirements and configuration.
I forwarded the KB article.
Thanks!