The Biztalk assembly needs a strong name key, if you can deploy your assembly then it has a strong name key.
Did you restart the Biztalk Host Instance after deploying the assembly containing the pipeline.
It is possible that the Host instance has loaded a previous version of the assembly, and although the configuration specifies the SampleTDPO3.ReceiveTDPOFlat pipeline, it does not exists in the already loaded assembly.