TraceLevel::notrc
An error occurs because the value of the TraceOn option is such that tracing is never turned on during this evaluation:
Trace[2 + (Print["trace level = ", TraceLevel[]];2), TraceOn -> All]This shows a value for the TraceOn option that causes tracing to be turned on during this evaluation:
Trace[2 + (Print["trace level = ", TraceLevel[]];2), TraceOn -> _]