Mark: I am afraid that defining a Dialer interface has no impact on whether the pppoe-client command is accepted or not.
Rizwan: Your suggestion would start a PPPoE server instead of PPPoE client on the interface. I don't believe that this is what atifsalahnour intends to do.
Atifsalahnour: Do not enter the pppoe enable command by yourself. Try to clear the interface configuration using the default interface fa0/0 command, and after that, try entering the pppoe-client dial-pool-number 1 on the Fa0/0 inerface directly without any preceding commands. If the command is still not accepted please let us know your exact IOS version and feature set. There is a possibility that the PPPoE client feature is not supported.