include fstream
this should fix:
/build/atnight/localbuilds/nightlies/Athena/main--ACTS/athena/MuonSpectrometer/MuonPhaseII/ActsMuonDetectorTest/src/MuonDetectorNavTest.cxx:46:25: error: variable 'std::ofstream os' has initializer but incomplete type
46 | std::ofstream os("event_" + std::to_string(eventNumber) + "_Steps.obj");