Upgrade to firmware 9.21.02.0014 or higher.
Release notes state, in the 'Changes in 9.21.02.0014' section:
wns0012719 Corrected the base license and capacity license for C35 wireless appliance to match the datasheet
Original firmware 9.21.01.0179 incorrectly supports a base of 25 APs, and AP Capacity upgrade types WS-APCAP-25 and WS-APCAP-100.
Corrected firmware 9.21.02.0014+ supports a base of 50 APs, and AP Capacity upgrade types WS-APCAP-1 and WS-APCAP-16.