What AOS version? Also if you run the following. While 124 may be allowed on your controller, the specific AP types also have their own allowed channels, of which 124 doesn't appear to be allowed on the AP-124. For me this is on AOS 6.4.4.6
(Aruba7005) #show ap allowed-channels country-code US ap-type 124
Allowed Channels for AP Type 124 Country Code "US" Country "United States"
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PHY Type Allowed Channels
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802.11g (indoor) 1 2 3 4 5 6 7 8 9 10 11
802.11a (indoor) 36 40 44 48 52 56 60 64 100 104 108 112 116 132 136 140 149 153 157 161 165
802.11g (outdoor) 1 2 3 4 5 6 7 8 9 10 11
802.11a (outdoor) 36 40 44 48 52 56 60 64 100 104 108 112 116 132 136 140 149 153 157 161 165
802.11g 40MHz (indoor) 1-5 2-6 3-7 4-8 5-9 6-10 7-11
802.11a 40MHz (indoor) 36-40 44-48 52-56 60-64 100-104 108-112 132-136 149-153 157-161
802.11g 40MHz (outdoor) 1-5 2-6 3-7 4-8 5-9 6-10 7-11
802.11a 40MHz (outdoor) 36-40 44-48 52-56 60-64 100-104 108-112 132-136 149-153 157-161
802.11a (DFS) 52 56 60 64 100 104 108 112 116 132 136 140
(Aruba7005) #