This error message is not clear and may come from a variety of sources. Here is one:

lemma L() {}

method test() 
  ensures L(); true

which produces

ERROR_PostconditionLemma.dfy(4,15): Error: this symbol not expected in Dafny
1 parse errors detected in ERROR_PostconditionLemma.dfy

The error message points to the token true as the unexpected symbol. true is definitely a legitimate symbol in Dafny.

The problem is that the ; in the ensures clause is seen as the (optional) semicolon that can end the clause. Thus the true is interpreted as the (illegal) start to a new clause (or a { to start the body).

The correct way to include a lemma with the postcondition is to use parentheses: `ensures (L(); true)