I've come across this specific issue and that particular error message before. What I noticed was that for an LACP port-channel, you cannot have one member port configured for CTS and another port configured without it. Here are the steps I had to perform to configure CTS on port-channels that were already in production:
1 - Disable port <a> on 9500
2 - Disable port <a> on router (ours was an isr4431, but same procedure applies)
3 - Remove both ports from their respective port-channels
4 - Configure CTS manual on both ports
5 - Configure new IP in a different subnet; also configure anything else needed for routing to converge over the individual ports
6 - Enable port <a> on 9500 and and port <a> on router
7 - Wait for routing to converge
8 - Disable port <b> on 9500
9 - Disable port <b> on router
10 - Remove 9500 port <b> from port-channel, configure with CTS manual, and then re-add to port-channel
11 - Configure CTS manual on the router port <b> (didn't have to remove and re-add from port-channel)
12 - Enable 9500 port <b> and router port <b>
13 - Wait for routing to reconverge on the port-channel with CTS enabled on their member ports
14 - Disable port <a> on 9500 and port <a> on router
15 - Remove IP address (and any routing config) from 9500 port <a>, configure CTS manual, and configure for port-channel
16 - Remove IP address (and any routing config from router port <a>, configure CTS manual, and configure for port-channel
17 - Enable 9500 port <a> and router port <a>
18 - At this point all ports should be members of their respective port-channels and have CTS enabled
Needless to say, this was not a very fun endeavor but worked successfully for me at one of our new sites. BTW, we used this same process with layer 2 port-channels down to our access switches without the need for configuring IPs and routing on the <a> ports while running in individual mode and let RSTP sort out which ports were used while doing the switchover.
