Sorry if this question as already been asked but when i was flying FL180 or 18,000ft I was off about 100ft and ATC kept telling me to climb and maintain FL180. When I was there he kept telling me to maintain FL180 which I did but kept getting that message from ATC. What was I doing wrong? Alt was set at FL180

what was the altimeter set to? local pressure or 29.92?jd

JDThe pressure was set to local pressure since the check altemiture didnt sound. I have flown this FL before and never had to check it. It has always been above FL180.

if you are told to fly a flight level, the altimeter has to be 29.92jd