Command rejected: Po1 not a switching port on Cisco Catalyst 4500

check the etherchannel status

SW1#show eth sum
Flags:  D - down        P - bundled in port-channel
        I - stand-alone s - suspended
        H - Hot-standby (LACP only)
        R - Layer3      S - Layer2
        U - in use      f - failed to allocate aggregator
        M - not in use, minimum links not met
        u - unsuitable for bundling
        w - waiting to be aggregated
Number of channel-groups in use: 1
Number of aggregators:           1
Group  Port-channel  Protocol    Ports
------+-------------+-----------+-----------------------------------------------
1      Po1(RU)          -        Gi2/6(P)   Gi2/10(P)

see.. RU . it's a layer 3 port
by default, if you start the machine and you see the name is router> then, all ports are layer 3 port
if switch> then all ports are on switchport mode
just change to layer2 are switchport to solve this

>switchport on both Gi2/6, Gi2/10
>switchport on po 1

show etherchannel again

SW1#show eth sum
Flags:  D - down        P - bundled in port-channel
        I - stand-alone s - suspended
        H - Hot-standby (LACP only)
        R - Layer3      S - Layer2
        U - in use      f - failed to allocate aggregator
        M - not in use, minimum links not met
        u - unsuitable for bundling
        w - waiting to be aggregated
Number of channel-groups in use: 1
Number of aggregators:           1
Group  Port-channel  Protocol    Ports
------+-------------+-----------+-----------------------------------------------
1      Po1(SU)          -        Gi2/6(P)   Gi2/10(P)

now SU already... happy trunking !!