I was following the guide on
When “Run CMake to create the Makefile and build the executable.”, i got the error. i didn’t do other changes, and haven’t know how to fix it.
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it seems to me that you are issuing the cmake .. command not in the usual directory. If your $GARFIELD_HOME=/home/mydir/garfield then you should issue the cmake .. command in $GARFIELD_HOME/build while you are now doing it in $GARFIELD_HOME/build/Gem/build, which makes you building the Gem example before having built Garfield.
Independently of the unusual build directory, there was a genuine issue with the neBEM build yesterday; please update to the HEAD of the master branch.