Charged pion decay in to Tau

Conservation of energy. The tau lepton is heavier than the pion.

 

In the CM of the pion, it will have [itex]E = m_\pi \approx 135 ~MeV[/itex]
Then decaying to something that needs at least (for its mass purposes) energy of [itex]E \sim M_\tau \approx 1777 ~MeV [/itex] is impossible.
That's something one has to check everytime he has a decay of the form : [itex] X \rightarrow A_1 + A_2 + ... +A_n [/itex]
That's not though the case of [itex] X+Y \rightarrow N- \text{particles}[/itex]