Hi
The command is as follow:
Altona_SW(config)#int Fa0/10
Altona_SW(config-if)#spanning-tree portfast
%Warning: portfast should only be enabled on ports connected to a single
host. Connecting hubs, concentrators, switches, bridges, etc... to this
interface when portfast is enabled, can cause temporary bridging loops.
Use with CAUTION
%Portfast has been configured on FastEthernet0/10 but will only
have effect when the interface is in a non-trunking mode.
Altona_SW(config-if)#
But the interface must be Access Interface, as you can see on the warning.